Firstly, yes capitalism has only been around for circa 400 years, because the emergence of capitalism and eventual adoption as the dominant mode of production was only made possible by colonialism and the influx of looted wealth and access to far off markets and slave labor that colonialism unlocked. Without colonialism and slavery, the primitive accumulation that was required for capitalism to take hold would not have been possible. Once you understand this you see that it is also the case that capitalism today could not sustain itself without imperialism and neo-colonialism.

Secondly, the US did not start out with oceans to both the east and west, it started out with only an ocean to the east. To get to the other one it had to wage a century of brutal genocidal wars against the native inhabitants of the majority of the continent, as well as against other European colonial powers and colonial states like Mexico. The US as it exists today was established through constant war, theft, broken treaties and the murder of untold millions. It was not so much luck as the fact that from its very inception it was a uniquely murderous society that had no limits whatsoever in what it was willing to do to fulfill its expansionist goals with almost messianic fervor.

The same mentality still drives its imperial policy today and it acts with the same total lack of respect for any agreements made with anyone else, whether ally or enemy. From the very beginning it has viewed the willingness of others to enter into treaties with it as a weakness to be exploited at the first opportunity. Unfortunately too many peoples have made the mistake of assuming that just because they were acting in good faith and being rational that the US would do the same. It has taken the world a long time to understand that the US as a society is just collectively psychopathic and agreement-incapable.