The run-up to Pearl Harbour involved various different events, deliberations, et cetera, but it is not unreasonable to say that Japan attacked America because Japan's leaders feared the possibility of America going to war with them. However, as you point out, that possibility for war arose from the fact that Japan was pursuing a policy of aggressive imperialism in East Asia.

Throughout the early 20th century, the Japanese Army expanded its hold in China by exploiting military "incidents" to justify claiming more and more land. In 1931, a local commander in the Kwantung Army (the Japanese Army group in Manchuria) engineered a false flag attack on a Japanese-owned railway in Manchuria, creating the so-called "Mukden Incident," which was used as a pretext by the Kwantung Army to invade the entirety of Manchuria (which was turned into the puppet state of Manchukuo). In 1937 the "Marco Polo Bridge Incident" - a small skirmish between Japanese and Chinese forces caused by the disappearance of a Japanese soldier (who later turned up to his unit again) escalated into an all-out war between Japan and China. This escalation is a fascinating topic, but not really the point of this answer. The point is that by the late 30s, Japan was indeed in a state of all-out war with China.

It would be a mistake to see the US as the ally of China throughout the entirety of these developments, however. Indeed, the US was willing to supply Japan with all sorts of raw materials needed for the upkeep and expansion of their armed forces throughout much of the 30s (something that caused quite a bit of controversy in American newspapers). Events such as the Rape of Nanking, however, and the general expansion of the scope of Japanese Imperialism in both China and Indochina (Japan sent troops into Vichy-controlled French Indochina in 1940), pushed both the American public to oppose Japanese efforts in a more meaningful way - namely, through the breaking off of commercial treaties and the embargo of materials such as oil, steel, and rubber.

Japan has the misfortune of being very poor in natural resources of her own, so that US embargo (which was joined by Britain and the Netherlands) put the war effort in China in jeopardy. The Japanese Army might not have been terribly mechanized compared to some other armies of WWII, but oil was still quite obviously a necessity for conducting an overseas war of that scope, as well as maintaining a first-rate navy and the economy to support this all. Negotiations were attempted, but the Japanese military was simply unwilling to give up their gains in China (America's condition for the lifting of the embargo), so war was the only recourse for the Empire.

British and Dutch holdings in Southeast Asia would have been appealing targets in 1941 - not only were they rich in natural resources, but the war in Europe meant that they were fairly unguarded. However, the US had a colony of her own in the region - the Philippines. Given that the US had been taking a strong stance in opposition to Japanese expansion both diplomatically and economically (and even militarily - the US started selling weapons to China for low prices), Japanese war planners thought it highly likely that the US would go to war with Japan even if Japan only attacked the British and Dutch, and if the US decided to do just that, the Philippines would serve as a rather sizeable "unsinkable aircraft carrier" - well placed to interfere with Japanese convoys going to and from the East Indies and Malaya. Rather than let the US go to war with Japan on her terms, the Japanese settled on attacking the US first - essentially, starting the war on their terms instead. The hope was that they would be able to wear down the US Pacific Fleet in the initial strike (Pearl Harbour) and as it moved across the Pacific, before defeating it in a decisive battle and which would bring America to the negotiating table - basically, the idea was that military success would cause America to agree to recognize Japanese holdings in the region in exchange for peace. The Japanese, of course, severely underestimated the US's will to carry out the war to its end, but that's another subject.

So yes, Japan attacked America because they feared that America would go to war with them. However, that fear was caused by America's response to Japanese acts in China and Indochina that can only be described as blatant imperialism. It wasn't a case of "America really hates Japan for no reason and forced Japan into a corner" or "Roosevelt really wanted to go to war with someone," as some people will claim, it was a case of American foreign policy being forced into a strong anti-Japanese Imperialism stance by the actions of the Japanese Empire. The fear of that America was going to attack Japan was predisposed by the idea that Japan was going to ramp up their expansion, after all. In general terms, there idea that Japan and America would come to blows in the Pacific was an idea that existed well before the Mukden Incident, but if one looking at the actual build-up to the war, it is easy to see that Japan was not an innocent victim of this vast Asia-Pacific War that they started.

Sources: A Modern History of Japan by Andrew Gordon, The Modern History of Japan by William G. Beasley, Kaigun by David C. Evans.

The debate over whether the US was justified in dropping the atomic bombs on Hiroshima and Nagasaki is a whole other can of worms, and it's a subject matter of which I'm probably not as qualified to discuss about as some other people in this sub.

It has been a while since I have looked at this, so I am sure that others will have more to provide than me. As such, I will limit my comments to the attack on pearl Harbour and let others provide more information on Japans general conduct during the war and the issues regarding the atomic bomb.

Japan's "sneak attack", or "war crime" as you put it has undergone several distinct changes in its historiographical interpretation. The most commonly offered and oldest is the one that you seem to have argued. Primarily, it holds that an expansionist Japan launched a sneak Attack on Pearl Harbour in an effort to cripple the American navy, opening the way for Japan to solidify its new holdings in South-East Asia. This interpretation fit well during the immediate wartime and post war atmosphere which demonized Japan as a racial "other"--essentially portraying them as sub-human and without honour.

While this has been the most prevalent interpretation, there is another which puts the blame for the attack more on the Americans themselves. It argues that the US largely forced Japan to attack them by placing sanctions on the material necessary for Japan's economy and military to function (I.e. Scrap metal and aviation fuel). Some historians have gone as far as saying that top level Americans (especially FDR) knew that Japan would be forced to declare war and hoped to use this as an excuse to go to war in Europe against hitter and secure American hegemony in Asia.

What I find most interesting here is not which interpretation is correct, as theremin evidence to support both positions, but that they have each been dominant with the American public depending on the contemporary relationship with Japan. As Emily Rosenberg has pointed out, the Japanese have been portrayed as a sub-human villain conducting a "sneak attack" or a noble foe who launched a "surprise attack" at different times in different contexts. When the US needed a strong Japan in the 1950s as a bulwark against Communism, for example, they were portrayed as the latter. In the 1980s when the Japanese economy was outpacing the Americans, on the other hand, they have been portrayed as the former in popular culture and the media.

• Emily Rosenberg, Date Which Will Live: Pearl Harbor in American Memory (Duke Univeristy Press, 2005)

Some earlier threads discuss the difference between how WW2 is perceived by Americans and Japanese.

How do japanese historians, textbooks, teachers, etc justify/ portray the attack on pearl harbor?

How do the people of Japan view Pearl Harbor, WW2(vs. America), and the bombings?

Why did Japan 'need' to attack Pearl Harbor when they wanted to get the oil fields in the pacific, Indonesia, etc... ?

They don't necessarily answer your question but they might help you understand your friends position on the war.

I also want to mention that part of Japan's reason for attacking Pearl Harbor was to placate the IJN. The army's success in China was shifting power in Japan's military government to the Army and away from the Navy. The IJN wanted their own success to balance this. By itself this wouldn't be enough to launch the attack but it likely helped influence the decision.

I strongly recommend the book Japan 1941: Countdown to Infamy, by Eri Hotta. It's quite dense but gives very detailed background and context to the decision to go to bomb Pearl Harbor. Said context includes the development of Japan's political system since the Meiji Restoration, contemporaneous thinking about the Tripartite Pact, how the media depicted the "China Incident" to the public, American intervention in China, and particularly Japanese ambivalence about modernizing (considering the limited breadth of the project, Hotta is very ambitious and enjoyable to read!). She also considers the extreme bitterness over racism directed at the Japanese by the West, particularly the high-handedness and perceived arrogance of the Americans, and the idea that unlikely successes in the First Sino-Japanese War, the Russo-Japanese War, and the invasion of Taiwan may have persuaded the military to push their luck.

Hotta is particularly critical of Konoe Fumimaro's lack of assertiveness, and also highlights colonial opportunism that was at least in part a response to supply embargoes (essentially, that many believed the war was a distraction to the European presence in Southeast Asia and one they could take advantage of, expanding their colonial presence while replenishing supplies rapidly dwindling due to the war in China). Ultimately she concludes that cronyism, ineptitude, overrun militarism, and fury over America's lack of regard especially regarding China led to Japan's entry in the war.

Hotta is extremely critical of prewar Showa conservatism and that has led some critics to accuse her of victim-blaming. I personally interpreted her book as quite sympathetic to the Japanese public and very frustrated with government bureaucracy, however I do think this criticism is a valid one to bear in mind when reading.

(I hope I am not summarizing too much but Japan 1941 is the most in-depth book in English to cover this topic so I think you'll find it very interesting.)

It's important to remember in dealing with your friend the extreme deprivation that the Japanese public experienced during the war. Even in 1940 rations were very poor and army conscription had already started to become extreme, drafting men already late into their thirties. Whatever the lead-up to this was (well detailed in other responses you've gotten so far), American embargoes on Japan contributed heavily to domestic deprivation. Propaganda about inevitable success in China was already widespread, and took tight hold during WWII as well: that victory would be swift, easy, and above all, just. Loyalty unto death was a well-established cornerstone of the military and became one of the public's in relation to the war effort as well. Hotta recounts the tragedy of many of Japan's children, who were evacuated (quite haphazardly) from major cities only to face starvation, death en route to evacuation sites, or even to be returned safely home only to die in later air raids. (This is all cited from Hotta.) I realize fiction recommendations are not par for the course for this subreddit, but two examples of how individual Japanese people confronted extreme deprivation during and immediately after the war are in the film Grave of the Fireflies and Kazuo Ishiguro's novel A Pale View of Hills. There is a lot of controversy in how Japan considers (or doesn't) its own past aggressions but there really can be no question that the Japanese public suffered enormously, and there is a lot of blame to spread around.

As for the bombings of Nagasaki and Hiroshima, it is enormously reductive to blame the military leaders "who brought them into this." I recommend some of the following sources:

Robert James Maddox's The Biggest Decision

J. Samuel Walker's Prompt and Utter Destruction

Gar Alperovitz's The Decision to Use the Atomic Bomb

Ronald Takaki's Hiroshima: Why America Dropped the Bomb

These sources all give some different views on the bomb and some are quite controversial, but the list gives some impression of just how complicated it is to attribute blame for the bomb.

I think one of the first things you can do to try to remove bias is not only see things in a dichotomy of "us" and "them." In a sense, you are partaking in the same kind of reasoning as your Japanese friend, and therefore perhaps just as wrong about the war. Others have already posted so I will just say, there are many opinions you can find concerning some of the actions in the war, and I find it best to just be conscious of that fact, rather than just coming to broad conclusions. Just to give an example, I know that for many Japanese war planners the embargo was viewed as an act of war. Likewise, some historians have written that Japan would have surrendered upon a combined Soviet and U.S. invasion. Concerning imperialism, we can all agree its bad, but is it wise to demonize the Japanese and not treat the Americans, British, and Dutch the same? My point is that bias and a simplified worldview does not help to arrive at some semblance of truth.

The book War Without Mercy: Race & Power in the Pacific War by John Dower, is a valuable read on this topic. The book explains the dehumanization of the enemy on both sides of the Pacific Conflict, and describes Japanese racial theory. In short, the Japanese had a concept called the South-east Asian Co-Prosperity Sphere. The Sphere was basically a fascist and racist imperial scheme of oppression of the Pacific Islands and the continent, that placed the Japanese at the center, administering the empire. The Rape of Nanking is a good example of the kind of horrors that the Japanese inflicted on the continent.

The United States and Japan had both made preparations for war over the Pacific at some point, in speculation that oil thirst and control would drive them into conflict. The way the western powers treated Asia during the 19th century didn't really set a good example for Japan, who sort of formulated their racial theories as a reaction to European racism.

Really I just think the book is relevant to your question and I haven't seen it recommended here.

Facts don't matter, the winner or dominating power dictates what was right or wrong at the end of a conflict and the myths lives on almost forever. After that we spend decades debating about who was guilty and what happened.

Check this thread about winners prosecuting one of their own as war criminals. Barely a few minor conviction, going back to ancient Greece.

Once things have changed and empires crumbled, it takes time and self criticism to admit that some countries (ours especially) committed horrible crimes. Even now a lot of people here in Europe won't admit that colonial empires were terrible things.

That being said, I think it's almost impossible to talk from within the US or the big european countries about war crimes and imperialism in a decent way.