Admittedly this stuff is rather complicated and open to tons of interpretation. In the current case in Ukraine, the US and other countries are supplying Ukraine with weapons. So without a doubt they are supporting Ukraine. However, it is not US "minds" deciding exactly on how those weapons are used, although the US is giving some guidelines to limit their offensive employment. That's on President Zelensky and his generals. No Americans are actually killing Russians in this regard. At least in terms of how a nation views its own involvement, its a pretty big difference. But just kind of how the politics play out it is possible to do so and kind of distance yourself from the war execution. WWII is a very good example in that the US supplied lots of weapons to the allies before entering the war "directly" at the end of 1941. Proxy wars are a thing, and this is one of the ways they are carried out.