The fourth decade of the 20th century saw many developments in both tanks and anti-tank weaponry, and all armies knew they needed both for any future war. One very good anti-tank gun was the 37 mm model made by Bofors of Sweden, which was manufactured under license by several countries including Poland, where it was named the wz.36. When Germany and the Soviet Union invaded Poland in 1939, there were about 1,200 of this gun in service with the Polish Army, and it could defeat any tank either of these countries could deploy. After Poland was overrun the gun was available to the conquerors, but as with all other 37 mm weapons it soon became obsolete as new tanks were produced with thicker armor which it could not penetrate.