Whoever, within the United States, knowingly begins or sets on foot or provides or prepares a means for or furnishes the money for, or takes part in, any military or naval expedition or enterprise to be carried on from thence against the territory or dominion of any foreign prince or state, or of any colony, district, or people with whom the United States is at peace, shall be fined under this title or imprisoned not more than three years, or both.
However, The USA has provided significant Security Cooperation with Ukraine with its war against Russia including investing billions of dollars in equipment, training, advisory services and advisory support etc (and rightly so).
Unless I am very much mistaken, the USA is currently at peace with Russia so its support for Ukraine, to me, seems to fall within the fairly broad definition of an enterprise against a "foreign state" and/or against "people with whom the United States is at peace". Presumably this latter point would include Russians within Ukraine.
My assumption is that the "whoever", as the subject of 18 USC 960, refers exclusively to non-governmental activities but is this expressly defined anywhere, such as in statute, case law, or by executive order?