Isolationism doesn't mean never projecting military power, it means not getting involved in alliances that don't serve, or barely serve our own interests.
George Washington's farewell address addressed this:
"The great rule of conduct for us, in regard to foreign nations, is in extending our commercial relations, to have with them as little political connection as possible. Europe has a set of primary interests, which to us have none, or a very remote relation. Hence she must be engaged in frequent controversies the causes of which are essentially foreign to our concerns. Hence, therefore, it must be unwise in us to implicate ourselves, by artificial ties, in the ordinary vicissitudes of her politics, or the ordinary combinations and collisions of her friendships or enmities."
Thomas Jefferson said that one of the "essential principles of our government" is that of "peace, commerce, and honest friendship with all nations, entangling alliances with none."
Monroe:
"In the wars of the European powers, in matters relating to themselves, we have never taken part, nor does it comport with our policy, so to do. It is only when our rights are invaded, or seriously menaced that we resent injuries, or make preparations for our defense."
Seward:
"defending 'our policy of non-intervention—straight, absolute, and peculiar as it may seem to other nations'...[t]he American people must be content to recommend the cause of human progress by the wisdom with which they should exercise the powers of self-government, forbearing at all times, and in every way, from foreign alliances, intervention, and interference."
Here's a Wiki on it:
https://en.wikipedia.org/wiki/United_States_non-intervention...
You seem to be arguing that the US projects military power in it's own interests. That's undeniable, but claiming that as a counterpoint that we weren't isolationist in the 19th century is not what isolation means. For recent examples: invading Afghanistan was for our own interests; we were attacked by a group sheltered by their government. Iraq was not. Syria is not. Balkans in the 1990s was not. Panama was because of the canal and trade routes. WWI and WWII ultimately was in our best interests but it wasn't obvious at the time, particularly early on, and the cost was immense. Right now, we are so intertwined in alliances that if just about any country goes to war outside of sub-saharan Africa, we are automatically at war.