Poland, Lithuania and Ukraine are to form a combined brigade in a bid to participate in UN and EU-sponsored peacekeeping missions.
The agreement for the joint army was signed by defence officials from the three countries during recent talks in Brussels.
Poland's Deputy Defence Minister Stanislaw Komorowski said that the agreement reflected the country's support for Ukraine.
"This is also proof that Ukraine is taking its desire for closer cooperation with members of the EU and Nato seriously," he said.
The new peacekeeping brigade intends to set up an alternative centre of military consolidation for Western European projects.
The move by EU members Poland and Lithuania comes two days before an EU summit with Russia, which aims to increase cooperation with Moscow.