I think part of our success, and maybe like you mentioned it would have only slowed us down not stopped us, is our history. How much does America being colonized by only a few nations play into effect? if more nations had gotten involved that is more conflict to start. more chances that a nation hangs onto their territory longer. As it was culturally we have been limited by the other major colonization. Canada and the french have never wanted to be part of the US, Spain/Mexico we defeated and or conquered a number of times yet we never really pushed south. The reason we got Alaska from Russia is because they didn't colonize the area. What if the Chinese had established a colony in California. most of our divergent ancestors happened under British or our own control and we have never really been pushed by another American power. Look at Europe and there have been so many powers that have defined each nation that they are doomed to the division. The EU looks like it might fall apart or scale back. We had to fight two wars to maintain independence, and even still we became closely tied with our mother nation, another major "mother nation" player would have complicated things immensely. so the power of the USA is just as dependent on our history and the details behind our founding as it is the geography and natural gifts.
I see what you're saying, but, when you think about it, this history you speak of is dependent upon geography. Geography dictated that the only two powers capable of really having any influence (maybe outside of Japan, which was secure as an island state from the Asian mainland) were Britain (secure island state from European mainland, and, therefore, seaward-looking) and France (not as secure, but still a sea power). Geography dictates that it's no coincidence Britain played the largest role on the North American continent. France was too concerned with continental affairs, as a part of the European continent, while, secure, Britain was nonetheless too occupied by European continental affairs as well, these affairs being far more crucial to its security and welfare than those occurring in North America. Even with Alaska and some holdings on the California coast (which I've visited now, as a Californian - yes, tis true), Russia's power and interest base was too far away and too disinterested. Similarly, China was too entangled by Eurasian affairs and security interests to worry about the North American continent.
another argument I will make is how geography would have extremely hindered America had it been colonized from another direction. From the north is too cold, short summers, if Britain has started in Canada and moved south it would have taken a much longer time, allowing for more players. The Spanish tried from the south but were limited by the harshness of their settled territory. Florida and Louisanna were undeveloped pits for most of history. Arizona and Southern Cal had populations but extremes of water would have limited development FROM those places. Same if we were settled from the west, no water in southern cal, poor summers in the north in Oregon and Washington, possible but much more difficult than settling in Virginia, and even then there were set backs. Texas could have worked, maybe, because it has all of the extremes and so there are stretches in between those extremes were development would have been possible, but you would have quickly run into the same issues mentioned elsewhere. So I guess I am saying the British/we got lucky they landed in Virginia and not Florida; the geography they faced was nicer and allowed quicker development than the other nations found.
I've thought about this as well, ever since I learned how crucial geography is in determining the destinies of nations and peoples. Of course it is not completely determinative, nor is geography a matter of conscious decision. It just is. Without a doubt, the history of the North American continent would be very different had secure seafaring cultures sprout up in East Asia, instead of in Western Europe. While it's probably not an accident if you could go back and have the omniscience to look at every detail of human history and land relationships, suffice it to say that it was merely a coincidence and a happy accident that Europe, particularly Britain sat nearly the same latitude (and technology and cultures tend to move on a west-east orientation due to climatic similarities, rather than north-south) as present-day America and Canada. That granted, it's no happy accident that Virginia was chosen as THE official spot by the British. Once again, geography (and climate), and it just happened to play in Britain's (and later, most notably, in our) favor, as opposed to the other colonial upstarts. Chances are, if you could wind back the colonial clock, excise British involvement in North America, and see what happens, the continent probably would never have taken off. No other spot was more suitable for a future "superpower spawn" than the US mid-Atlantic.
Marx was right about one thing at least: history is the condition of material being. However, he was wrong about the material. History is not the condition of economic relationships; rather, history and economic relationships as an extension, are the conditions of geography. And I might more aptly use geography/climate as the correct term. Geography, climate, and demographics, these are the most important factors that influence a nation's history and trajectory, although even demographics are subservient to the other two factors.