I think that a sense of American nationalism or exceptionalism might have been a reason for expanding interests abroad. It seemed that a combination of industrialization, the acquisition of wealth, and the belief that America was "chosen" (a la O'Sullivan's Manifest Destiny) seemed to contribute to a bold push to expanding interests outside of America. The combination, or some would say "collusion," of economic and political interests led to greater interest abroad.
If you've ever seen "The Last Samurai", you get a up close look at the way in which the Americans sought to sell weapondry and war tactics training to the Empire of Japan. The Japanese wished to become a major world player, but their military was a bit less than capable. So, Americans were willing to sell their American-made rifles to international buyers in exchange for a good price. Because of the wild west and the need to constantly upgrade weapons in order to efficiently wipe out any enemies on the frontier, weapons manufacturing was one way in which the American export product was successful.
Definitely with trade to other nations. Import and export of resources and products...food items, materials, spices, tea, ...all were valued commodities.
Till nearly the close of 19th century USA did not take very much interest in expanding its power in the world, or in interfering with wars of other countries. Till about 1890 it was weak militarily. It did not have much influence or interest in international politics. However things began to change in 1890's.
USA aided the rebels in Cuba fighting Spain, which ruled Cuba at that time. In 1998 USA declared war on Spain and defeated it. IN 1901 Roosevelt became the president and adopted the policy of "speak softly but carry a big stick". This meant combining diplomatic efforts with military power. IN accordance with this policy USA was able to secure right to dig Panama Canal by threats to use force. In 1905 it took over the finances of Dominion Republic to keep it stable and protect it from European intervention. With these and other similar actions USA emerged as a World power.
You are right--weapons and Defence equipment sales were a major booster to American economy after world war--then on it has been technology of all kinds and creating dependance on US technology companies and ofcourse Financial groups--and once US slipped--see what has happened--
It will be interesting to watch how Science and technology can bring in alternative energy solutions to counter Petro dollars and Arab empires--
In eighteenth century Imperialism was the name of the game of the Powerful nations like Britain,Dutch,Spain,Portugal and France--but after second world war America became a nation with Technology as its strong weapon--They didnot follow other countries example of expanionism and rather looked for markets abroad for their Arms,Aeroplanes and Steel/Railways--and see its results--American technological domination on the World as a whole--