Germany, 1942-1945 (312 download, please be patient!)
The Nazis in WW2 did not have a lot of resources, so throughout the war they often reused captured materials instead of building their own. This can also be seen in the design of the Hetzer. This design is based off of the gun-shield and chassis of the PaK 38, but its gun has been replaced with a 75mm M1897, captured from France. The M1897 was the first modern artillery piece, and served with distinction during WW1. By the time of WW2 it was still in service with the French Armed Forces and thus were captured when she fell.
When the T-34 and KV-1 were encountered during operation Barbarossa, the Nazi invasion of the Soviet union, none of the german AT guns could penetrate them. This led to the old M1897 guns being equipped with new carriages and HEAT shells. These guns were sent to the front in order to serve as emergency guns to stop Soviet Tanks. Interestingly, the M3 GMC uses a gun that was derived from the M1897 as well, despite being on opposite sides of the war.