American security depends on the balance of power in Europe and Asia. If a single power were able to dominate the continent (France under Napoleon, Germany under Wilhelm II or Hitler, the Soviet Union), it would also be strong enough to threaten the US.

As Jefferson put it: the enduring interest of the US lies in preventing the entire force of Europe from being wielded by a single hand.

George F. Kennan, American Diplomacy 1900-1950 (1951):

Today, standing at the end rather than the beginning of this half-century, some of us see certain fundamental elements on which we suspect that American security has rested. We can see that our security has been dependent throughout much of our history on the position of Britain; that Canada, in particular, has been a useful and indispensable hostage to good relations between our country and British Empire; and that Britain's position, in turn, has depended on the maintenance of a balance of power on the European Continent. Thus it was essential to us, as it was to Britain, that no single Continental land power should come to dominate the entire Eurasian land mass. Our interest has lain rather in the maintenance of some sort of stable balance among the powers of the interior, in order that none of them should effect the subjugation of the others, conquer the seafaring fringes of the land mass, become a great sea power as well as land power, shatter the position of England, and enter--as in these circumstances it certainly would--on an overseas expansion hostile to ourselves and supported by the immense resources of the interior of Europe and Asia.

Hence the enduring US alliances with NATO (especially Britain) and Japan.