What do you mean by "enter WWII"?

Japan was involved in the war since the 30s. Arguably as early as 1931-32 (border conflicts w/ the Soviets, early fighting against the Chinese as far south as Shanghai), or more conservatively 1937 depending on how you want to mark the start of the Second Sino-Japanese War. That's two years before the war began in Europe.

What happened later is that these two regions combined as Japan allied with Italy and Germany, after Germany stopped assisting Republican China, related to the fact that the Soviets and Japan continued to have tensions over Manchuria.

Japan didn't enter WWII so much as they began WWII.

I would point you to Chapter 17 of Joseph Maiolo’s Cry Havoc in which he depicts the logic of arms races and how it influenced Japanese decision making. (For the analysis, skip to the block quotes at the bottom of this answer) From pg 387 and following he details the pressures that the Japanese found themselves subject to: a war in China that wouldn’t seem to end, a scarcity of resources that could be “solved” by either moving north (into Russia, especially after Barbarossa began) or south (into Indochina and the Dutch East Indies). The army favored the northern plan, while the navy favored the southern approach. Russia failed to collapse as predicted, making the northern plan less attractive. The Dutch East Indies were a great source of oil, but the Japanese concluded that the British were highly unlikely to simply accede to Japan taking that territory. As such, an assault on the British bastion in Singapore would remove a way for the oil to be interdicted—but the “old guard in top posts in the navy”also concluded that the British and Americans were “indivisible.” (Pg 388)

The critics of the old guard—bellicose staffers who in the 1930’s had backed the anti-arms control fleet faction against the treaty faction—rejected this sort of defeatist thinking. If you accepted that war against the United States was unwinnable, then you handed the army predominance in national policy making and Washington a veto over foreign policy. With the Americans set to build a gigantic two-ocean fleet and vast air armadas, and the alluring but deceptive exemplar of Hitler’s Blitzkreig in their brains, the navy’s ultra-hawkish staffers began to speak of the logic of fighting the United States in a lighting action sooner rather than later. (Pg 388)

The navy won the debate over the army (and got a much larger amount of the steel allocations for their fleet-building programs), but delayed in taking action as diplomatic attempts to gain their objectives continued. It was hoped that a settlement could be reached so that a conflict with the US could be avoided.

Admiral Kondo Nobutake of the naval staff summed up his service’s awkward problem: “The navy has not yet completed its war preparations vis-a-vis the United States. They will be completed next April and Japan will stand a fair chance of winning on the basis of ‘quick encounter, quick show-down.’ But if the United States should turn the war into a protracted struggle, it will become very difficult for Japan. On the other hand, the United States is rapidly building ships and, in the future, discprepencies in the naval ratios will widen and Japan will not be able to catch up. In this sense, today is the most advantageous moment for starting a war. (Pg 389)

Maiolo then details the diplomatic hopes of firebrands within the Japanese command.

[T]he Germans would help compel the Dutch authorities to agree to Japanese demands for more oil; Berlin would broker a pact between Moscow and Tokyo; Chaing Kai-shek would come to his senses and pleade for peace terms: the Americans, always “level-Headed,” So Matsuoka said, would also see reason; if they did not, the Japanese Empire, allied with the Nazi regime and perhaps even supplied with Soviet raw materials, could face a long war against the United States anyway. (Pg 389)

None of the above happened. When Japan moved into northern Indochina (September 1940) the US cut off exports of scrap metal and aviation fuel to Japan. The English reopened the supply route to China through Burma, Chaing Kai-shek did not come to terms, and the Dutch wouldn’t budge. Neither would the Americans:

[T]he Americans wanted Japan to denounce the Axis, pull out of China and reverse the southward advance; for most Japanese officials that meant defeat. What they wanted was a truce that would allow a renewal of imports. (Pg 392)

The Japanese economy was reliant on those imports. Without imports the Japanese economy would grind to a halt.

The economy depended on trade, and because of falling earnings from exports, the war with China, and the economic embargo spearheaded by hard-liners in the re-elected Roosevelt administration, imports would soon nose-dive. In 1940 under the mobilization plan Japan’s import capacity for iron, steel, non-ferrous metals, fuel and food had been 2.6 billion yen; in 1941 it was eight million yen. (Pg 391)

In July there was another conference of Japanese commanders with the Emperor. The move into the rest of Indochina (revealed by code breakers) and fears over a Japanese attack on the Soviets or British resulted in freezing Japanese assets in the US. This “resulted in a total oil embargo on Japan” since neither the US nor the Dutch East Indies would sell oil any longer (and those two sources were the two largest contributors). (Pg 393)

Here is when we get into the direct logic for war against the US and Britain.

From September to December 1941 the compelling language and do-or-die logic of fast-escalating arms races and total war gripped the top decision makers in Tokyo. Under the oppressive sense that time was running out, hope and fear merged into a decision reached by consensus, a decision that then flowed into fateful action.

At an Imperial Conference on September 6, the emperor heard the case for plunging into all-out war against the United States, Britain and the Dutch was officially aired before the emperor. [sic] General Suzuki Teiichii, the new director of the Cabinet Planning Board, gave the most important presentation. “At this stage our national power with respect to physical resources has come to depend entirely on the productive capacity of the Empire itself…and upon vital material stockpiled thus far.” He continued, describing a situation similar to Italy’s the year before, and the sort of rationale for reckless action that had swayed even normally prudent Germans such as General Thomas to back Operation Barbarossa: “Therefore, as a result of the present overall economic blockade impose by Great Britain and the United States, our Empire’s national power is declining day by day. Our liquid fuel stockpile, which is most important, will reach bottom by July of next year…I believe it is vitally important for the survival of our Empire that we make up our minds to establish and stabilise [sic] a firm economic base.” Offered the choice between economic-military paralysis in less than a year or an admittedly risky southward thrust to seize the rich sources of oil, bauxite, nickel and rubber of Southeast Asia, the conference ratified a resolution to pursue talks with Washington and at the same time to get ready for war by mid-October. (Pg 394)

There was a final conference in November. After exhausting diplomatic approaches, Tojo reached his conclusions.

”If we enter into a protracted war, there will be difficulties,” the prime minister asserted:

The first stage of the war will not be difficult. We have some uneasiness about a protracted war. But how can we let the United States continue to do as she pleases, even though there is some uneasiness? Two years from now we will have no petroleum for military uses. Ships will stop moving. When I think about the strengthening American defenses in the Southwest Pacific, the expansion of the American fleet, the unfinished China Incident, and so on, I see no end to difficulties…I fear we would become a third-class nation after two or three years if we just sat tight (Pg 395)

In conclusion, Japan was unwilling to give up its war in China or its desire for empire and it would soon be unable to import enough resources to continue as they had been. While war against the US was a risk, the potential reward of being self-sufficient in resources and having their own empire was chosen over peace. It was an all or nothing gamble, and in the end they lost.

As always, followup questions by OP and others are encouraged.

First, you have to understand the geopolitical situation in the region:

1. Traditionally (and I mean for hundreds of years), China had been the cultural centre of East Asia. There is a reason why China is known as 中国, the middle kingdom. The cultures and religions of Korea, Japan and Vietnam all had some form of origin in China (Though the religions may not have necessarily originated in China). Naturally this would influence the way the states interacted with each other in the region (simplifying it China was basically the boss) This changed in the 1860s-1900s. Japan became the new regional power and had forced the Qing dynasty to sign the humiliating Treaty of Shiminoseki . Further underlining their ascension to power is their victory over the Russians in the Russo-Japanese war. This is crucial as not only had Japan showed their power in Asian politics BUT also in Europe. They had beaten the 'white men'. What did this mean geopolitically? There was a shift in paradigm. Japan became the centre. Now Chinese students were going to Japan to learn (both general education and military tactics), the famous Chiang Kai Shek studied in the Imperial Japanese Army Academy! Japan was going to extend it's influence in the region, whether China liked it or not and China definitely did not like it.

2. Moving onto the 1920s, what was going on in Japan? Things were not so smooth as there was disharmony in their government and armed forces (I'm sorry, I don't have much information on this other than the fact that the government slowly became pro-militarist as more military men took up seats in the government). Japan was facing an economic crisis due to the Great Depression and overpopulation was not helping. Furthermore, they had invested a lot into their armed forces and trust me, it is not fun to have no purpose. Imperialism seemed to be the answer to these two problems. Not only would they receive economic benefits from an economic zone under their control but it would also mobilise their armed forces, thus giving them a purpose. They had already followed the example set by the West and set up regions of extraterritoriality in China before. But they would now take it one step further. Expanding from Korea (They gained control over it after the Treaty of Shiminoseki and annexed it formally in 1911 I believe) they sent the Kwantung Army to 'protect their interests' in Manchuria. After the Mukden incident in 1931 (supposedly orchestrated by Japanese officers), they used it as an excuse to invade and occupy and set up the puppet state of Manchukuo. Now they had a resource rich region. They could exploit the region AND resettle their population, solving their economic crisis and overpopulation. This soon became their strategy in Asia and for the future. **They wanted to create a Great Asia Co-Prosperity Sphere, with them at the head of course. I guess this gives you some kind of hint of their ultimate goal? **

3. Now you might ask, why the hell didn't China do anything about it? Well the thing is, China was going through a very unsettling period to say the least. There was no single entity that could say that they ruled the entire country. The Guomindang (GMD) came the closest but in reality they relied heavily on the private deals they made with the warlords (warlordism, great topic to research). On top of this, Chiang Kai Shek was also undergoing a campaign to eliminate the CCP (1934ish). To the GMD, unification was key to a strong China, thus giving them a slim chance of battling the Japanese. I think we take it for granted now that nation-states are a given. But in China it was a long and arduous process, the search for a national identity. But I digress, moving back to the Japanese agenda. From about 1931-37, Chiang knew he had almost no chance of beating the Japanese and he needed to bide his time to modernise Chinese industry and armed forces. He knew that armed conflict with Japan was inevitable, Japanese aggression was escalating as well as anti-Imperialist (specifically Japanese in this case) amongst the people as well. Overall, Japan was taking advantage of the vacuum of political power in China and the people were not gonna take it standing down (I mean come on, a hundred years ago these Japanese were 'below' us, said the Chinese people.) Armed conflict loomed near.

Okay now that I have given some context to the period (albeit sloppily but hey, I'm typing this on my bed) lets look at I guess how the Second Sino-Japanese War started.

1. The immediate cause can be seen as the Marco-Polo Incident. It gave the Japanese an excuse to open fire and they did. But this was nothing much if seen on its own. Skirmishes between the two sides occurred now and then, why should this matter? It was because Chiang saw this as the straw that broke the camels back. He knew that he would lose legitimacy if he kept on allowing Japanese aggression to go unchecked. The people needed to see China retaliate. And so they did... BUT IN SHANGHAI!?!? THE BRIDGE EVENT WAS IN THE NORTH NEAR BEIJING!

2. Chiang saw Shanghai as where the battle needed to be. First, he could test the loyalties of the warlords under him (remember that point of lack of unity?) Second, Shanghai was the economic centre of China. The capital might've been in Nanjing and the diplomatic action there as well but Shanghai was where you needed to be if you wanted to make money. If Chiang could secure Shanghai, he could secure industry that would help his war effort. Another key point is this fight would have to be in the centre of the country. Manchuria? Meh they're not even 'Chinese', they're Manchus (the people that formed the Qing dynasty). Yea sure the Chinese people felt angry that Japan was taking land but perhaps not angry enough to go to war (remember now, though it might seem China is a homogenous place it really isn't. The languages Cantonese and Hokkein from the regions of Guangdong and Fujian are really different even though they are so close geographically. If linguistics is an important part of identity after the Westphalian model was introduced, this kinda shows how hard it is to unite the people of China). Yes this kinda undermines my previous point but I think it's important to know the complexities of governing China and understanding the feelings of the people. Rana Mitter pg 96

3. Sooo Chiang attacked in Shanghai... What did the Japanese think? Well according to Mitter it was in their interest to play down this conflict. They referred to many battles as ''incidents' that had flared up and would be damped down with a firm hand'. (Mitter 95) This is because Japan needed China to remain fragmented, allowing them better control over the regions they had influence over. A weak China meant more economic prosperity for Japan in Manchuria. If these battles escalate into a war, it would unite the people of China against a common enemy, them. I guess this answers your question of 'why did it seem like the Japanese 'stumbled' into the war'. To an outside observer, it might seem like the Japanese didn't think that their actions would create a large scale war.

Sorry for the really sloppy wall of text. I didn't put much planning into this and there isn't much structure, if any at all! I butchered many interesting points but please do read them in more detail in the books I mentioned below. Please do ask questions! I feel what I said might cause more questions.

Bibliography: For general Chinese background history: Spence, Jonathan D. The Search For Modern China. New York: W.W. Norton & Company, 2013. Print

For more information on the Second Sino-Japanese War and contextual information: Mitter, Rana. China's War With Japan: 1937-1945 The Struggle For Survival. London: Penguin Books, 2014. Print

^ Okay I'm not sure how to cite this, Allen Lane originally published it in 2013