During the Polish–Soviet War (1919–21), Hungary offered to send 30,000 cavalry to Poland's aid; however, the Czechoslovak government refused to allow them passage through the demilitarized zone which had existed between Czechoslovakia and Hungary since the Hungarian–Czechoslovak War several months earlier. The Romanian government took a similar stance, also refusing passage. When the Hungarians tried to send ammunition trains, Czechoslovakia again refused but Romania agreed if the Hungarians used their own trains. Hundreds of Hungarian volunteers fought on the side of Poland in the war, and some stayed in Poland after the war.