The negative economic impacts of what I think is pretty much the early stages of WW3 (like the sino-chinese war that went on for a few years before 1939) come from 2 main sources. Disruption in gas and agricultural products from Russia and Ukraine, and disruption in trade in the red sea.

The other comment or points out that the us is diverting productive labor into militarism from this war, but that has always been the case. The us wastes a shockingly high amount of its labor on garbage, but this won't show up on the gdp statistics because those only tell you how much purchasing power a country has (relative to others), not what that power is used on.

We’re sending older equipment in order to make more new stuff.

This is just MIC propaganda. Don't fall for overly convenient narratives.