Not to start a big discussion about WW2 but the US was passive until they were attacked. Over 2000 Americans were killed in Pearl Harbor. For a nation of its size and power, the US was decidedly un-imperialist up until then. Even after they'd beat the Nazis and Imperial Japan, they actually helped rebuild their economies instead of exploiting them. Granted, that might have been the smarter thing to do anyway and turned out a win-win. But it wasn't how most leaders thought at the time. Look at the Soviet Union and how they ended up oppressing the territories they "liberated". The Western allies also wanted to keep Germany down, as did some in the US government (see the Morgenthau Plan for example). Had they prevailed there might soon have been another war.

For a nation of its size and power, the US was decidedly un-imperialist up until then.

Its empire was never on the scale of the major European Powers. But by that point in time, it still maintained explicit colonial control over the Philippines, Puerto Rico, Hawaii (still fairly recently subjugated) and numerous Pacific islands. Along with the Panama Canal Zone (which had its own postcal code, CZ).

It also exerted considerable influence over the affairs of many nominally independent countries in the hemisphere (Cuba quite notably), and engaged in several major military interventions up until 1933 (Mexico, Haiti, Dominican Republic, Nicaragua). It also intervened substantially in the Russian Civil War, up until 1925, and was still engaged in wars of suppression against its indigenous population through the middle of that decade as well.

One could say its imperial project took a breather of sorts in the mid-1930s, and decided to rest on its laurels for a bit.

But "decidedly un-imperalist" it was not.