You guys are confusing several things. The import of WW2 "Lend Lease" and the various post-war and Cold War lent or gifted military aid is prohibited because the US Gov't technically still owns it.
Also there are some legislated protections for manufacturers from "free" guns that were being dumped into the US market in competition with existing manufacturers.
US guns (assuming not Class3) that were actually purchased by the owning country are perfectly legal for import to the US.
Also the Gun Control Act of 1968 made it illegal to import post 1898 manufactured guns which were owned by any military ~ hence the Canadian "happy time" from 1968-1986 when milsurp could not enter the US, but could come to Canada. The loop-hole was that police contract guns were still perfectly legal for import ~ creating a market for short mauser "gendamerie" rifles which is forgotten today....
In 1986 the "Gun Owners Protection Act" again made the import of obsolescent military firearms legal, which also coincided with the slow-down of imports to Canada...