First of all it is important to note one things: Japan was NOT an isolationist country. What is funny, is that people will point to the fact that Japan refused to trade with anybody... except that is categorically untrue. The Tokugawa Japan traded with Korea, the Ryukun Kingdom, the Chinese Empire (Ming/Qing Dynasties), the Dutch, the Ainu, various Malay states. They were hardly inward looking at all.

Now you might ask, well they banned all Europeans right (except the Dutch)? Well yes and no. What we are missing in this picture, is ironically the very people who are the exception to this rule. Yes, the Tokugawa Shogunate closed off trading to the Spanish, Portuguese, and other Europeans (with mixed results, as you did have British trade and the such), due to a fear of religious (Christian) revivalism. The biggest threat to the hundreds of years of peace, that prevailed after the end of the Sengaku Jidai, was the uprising of Christian Daimyo against the Shogun. So great was this fear, that Christianity was banned, and the states promulgating such banned religion, were also banned. This was specifically because of the missionary work of Franciscans, Dominicans and Jesuits that were sent there. Whereas the Dutch (and English) had no interest in pursuing missionary activity (partly why they were so successful in making local allies and expanding their influence in modern day India and Indonesia).

Now what about the Dutch? We need to look at the wider picture of the Indian Ocean and South East Asia, where the Portuguese and Spanish were quickly being outmaneuvered and outgunned by the Dutch, beginning in the early 17th century (thanks to the Flyute amongst other naval advances). The Dutch had no interest in seeing the Portuguese succeed in their trade ambitions, and monopolizing Japanese trade (but more importantly trade in the "Spice Islands") was therefore very important, and the Dutch, even more so than the Japanese, helped to curtail and end Portuguese trade to Japan, with their seizing of main Portuguese trade posts east of Goa (leaving only East Timor and Macau), and sinking of Portuguese ships.

During this period of flourishing trade with the Dutch, you had an outpouring of interest in the outside world, by an emerging Japanese middle class, of merchants and samurai, who had a lot of excess money, due to a budding inter-Japan trade (flourishing due to years of peace and investment). This resulted in "Rangaku", or "Dutch Learning". Japanese people studied medicine, canon founding, empirical studies, map making, Dutch language. Far from being "isolationist", 17th and 18th century Japan, was one of immense cultural exploration and education.

The end result was that Japan actually knew far more about the "outside" world, Europe for example, than Europe knew about them.

So where does this myth of Japanese isolationism come from? It stems from the Tokugawa Shogunates refusal to admit Americans into Tokyo Harbor in 1853, when Commodore Perry showed up. In an attempt to explain why Japan did not want to accept trade and harbour concessions with the Americans (who were really only interested in trade with Qing China), they created this historical myth of the "isolated" nation, saying this is the way it had always been in Tokugawa Japan. The Shogun was not interested in signing trade concessions to the Americans, outside of the terms that they demanded (ie. only trading in Dejima with the Dutch), and were especially worried after the First Opium War in China, of European colonial encroachment around their territory. Nonetheless this did not work, after Perry threatened the Shogun, forcing him to back down.

Sources: Inbound....

Japan hasn't traditionally been an inward-looking, isolationist nation; that popular impression is derived from the "isolation" (sakoku) policy that the Tokugawa shogunate implemented to ensure diplomatic stability and reduce potential threats to their rule. But recent historians have increasingly questioned this impression and instead emphasized how Japan continued to engage in trade with Korea and China and gathered information about the outside world through the Dutch. It should also be remembered that the Tokugawa shogunate only represents a short period of Japanese history; prior to that time the major hindrance to Japanese interaction with the outside world was their inability to fully integrate themselves into the Sinocentric world order, not any desire for isolationism.

But to address your actual question, yes, European influence was a decisive factor in the Japanese becoming expansionist. The Meiji oligarchs who controlled Japan as it modernized following the end of the Tokugawa shogunate believed that progress and development followed a unilinear course, that the Western powers represented what a modern state was meant to look like. To be a great nation meant having an empire and they very much wanted Japan to be a great nation.

There was a strong defensive element as well, as much as "defensive imperialism" may seem absurd. For Japan, acquiring an empire was directly tied to security in a way that it wasn't for the other colonial powers. The power vacuum in Asia that accompanied China's collapse made the areas bordering Japan open to Western encroachment which could potentially lead to Western advances on Japan itself. A Prussian adviser to the Meiji government famously described the Korean peninsula as a "dagger aimed at the heart of Japan" and Japanese leaders were very concerned that Chinese and Korean weakness would allow the Russians to gain control of Korea. All of the major Japanese imperialist acts of the Meiji period (the Sino-Japanese War, the Russo-Japanese War, and the annexation of Korea) were aimed at securing control of Korea. This thinking then proceeded to feed upon itself: once they had gained Korea, they then believed it was necessary to control Manchuria to ensure that their hold on Korea was secure, and so on.

There were a number of other factors as well, of course: prestige, greed, nationalism, etc.. Some believed in a Japanese version of the "white man's burden", that it was Japan's obligation, as the first Asian country to modernize, to show other Asian countries the way forward.

Source: Kenneth Pyle, Japan Rising: The Resurgence of Japanese Power and Purpose

A major factor in Japan's expansionism was their feeling of being threatened. They were angered by the unequal treaties they were forced to sign when Admiral Perry arrived, and were very conscious of the imbalance of power that caused it. They were also extremely aware of the ways that western nations were carving up Asia and colonizing it, and were worried that that would happen to them. The solution was to become powerful and become a colonizer rather than letting themselves be colonized. That's the kick in the pants that started Japanese expansion in the late 19th century. But it continued, and grew, because frankly, being a colonizer is a great money-making opportunity. It helped to fund Japanese economic growth. In the early 20th century, gradually, more imperialist politicians gained power and a whole ethos of expansion was solidified. The mantra that Japan was a small country (which you'll still hear people say today. But it's small compared to what? It's bigger than a lot of powerful European nations) was repeated, a justification for its need to expand. Emphasis was also placed on its poverty of natural resources--it needed to colonize and expand in order to get the natural resources it needed to fuel its growing economy and industrial revolution. Ultimately, a view of the Japanese as having basically a duty to rule over all of Asia and free Asia from western colonialist aggression was put forth. It wasn't really true, in the sense that they weren't looking to free other Asian nations, they were seeking to form the same lucrative colonial relationships with Asian nations that Europeans had.

the decisive shift away from isolationism is complicated, but it was largely caused by three factors:

1. the rapid decline of qing dynasty china, which had had hegemonic power over the far east since the 1700s. between 1839 and 1864, the dynasty was shattered by the massive taiping rebellion and two disastrous wars with the west, culminating in the unequal treaties, the legalization of opium, and the opening of chinese ports to western trade. the opium wars, in particular, had revealed the dynasty to be a paper tiger. during the opium wars, western powers could expect to fight and win battles against the qing even when at a huge numerical disadvantage. at the pearl river forts in 1856, for instance, three american warships successfully carried out an amphibious assault on the qing defensive fortresses while outnumbered 9:1. (for comparison, at the first battle of fort fisher in 1864, the u.s. navy had total naval supremacy, had an 8:1 numerical advantage over the confederate defenders, and still failed to seize the fort.)

2. connected to that is the total superiority of western tactics and weapons. the japanese were absolutely powerless to stop commodore perry and the black ships from entering into tokyo bay virtually unopposed in 1853 and forcing japan open at gunpoint.

3. the japanese ruling classes were totally aware of these developments, and did as much as possible to mitigate them on the principle of, "if you can't beat them, join them." one of the notable essays of this period, the anonymously-written "leaving asia", argued that japan should join the western ranks by emulating them-- in stark contrast to the chaos enveloping china and korea. the japanese were absolutely willing to borrow and adapt as many western influences as were necessary to maintain japan's place in the world. the ruling classes borrowed their constitution from the prussians, their naval doctrine from the french, and hired all kinds of experts in trade and industrial power from all across the west. (of course, it didn't hurt that the opposition had been thoroughly discredited and crushed by their unsuccessful revolt against the emperor, permitting the satsuma oligarchs to govern as they pleased.) the result was modernization, the likes of which the world has not seen before or since.

with the entry of japan into the ranks of the european great powers also comes the japanese adoption of the prevailing ideologies of europe at the time, which meant imperial expansionism. the logical places for that were china and korea.

donald keene, emperor of japan: meiji and his world, 1852-1912.

see also james mooney, dictionary of american naval fighting ships.