U.S. military forces in Europe started an exercise Tuesday that will put 28,000 troops from 26 countries near Russian territory for over six weeks, with a focus on the Balkans and Black Sea region.

European Command’s largest multinational exercise comes on the heels of an alleged Russian military exercise involving some 100,000 troops massed on the eastern border of Ukraine and in occupied Crimea. The long-planned American exercise will test infrastructure and interoperability in NATO’s southeastern flank, home to the former countries of Yugoslavia and the European Union’s poorest member states but also some that spend big on defense.