The parachutes dropped into Poland on Monday in bunches, carrying U.S. paratroopers and pieces of a powerful M119 Howitzer to be assembled on the ground. It’s part of a training exercise designed to keep U.S. and European troops sharp. It’s also something more: A piece of a broad military exercise spanning multiple countries on Russia’s doorstep.

Saber Strike 15, scheduled from June 8 through 19, is the latest iteration of an exercise that dates back to at least 2010. But it continues to grow, as countries on Europe’s eastern flank register concern about Russian aggression in Ukraine. This year’s version includes at least 6,000 troops from 13 countries, military officials said. That’s up from 4,500 troops from 10 countries last year.

The latest version of the exercise includes troops from Canada, Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Norway, Poland, Portugal, Slovenia, the United Kingdom and the United States. Germany, Portugal and Slovenia are the latest countries to join. Denmark and Canada participated for the first time in 2014.

In addition to Poland, the exercise includes operations in Latvia and Lithuania, countries that were formerly part of the Soviet Union and have joined NATO. Here are some photographs of the action, which includes a simulated bombing run by an A-10 attack plane, tank operations and a military assault: