FILE - In this Monday, June 4, 2018 file photo a U.S. marine takes  part in a landing operation during military Exercise Baltops 2018 at the Baltic Sea near village Nemirseta in Klaipeda district, some 340 kms (211 miles) west north of the capital Vilnius, Lithuania. Lithuania. The American military says it’s preparing a massive exercise early next year in Europe involving 20,000 soldiers from the U.S., the largest deployment across the Atlantic in more than 25 years for training. (AP Photo/Mindaugas Kulbis, file)

BERLIN (AP) — The U.S. military says it's preparing a massive exercise early next year in Europe involving 20,000 soldiers from the U.S., the largest deployment across the Atlantic for training in more than 25 years.

U.S. European Command said Monday the "Defender Europe 20" exercise from April to May 2020 will support NATO objectives "to build readiness within the alliance and deter potential adversaries." Eighteen countries are expected to take part in exercises across 10 countries, including Germany, Poland, Latvia, Lithuania, Estonia and Georgia.

It will also involve 9,000 more Americans already stationed in Europe and 8,000 European troops.

The military says the exercise "confirms that the U.S. commitment to NATO and the defense of Europe remains ironclad."

President Donald Trump has worried many NATO members with comments that the trans-Atlantic alliance is "obsolete."

FILE - In this Saturday, June 9, 2018 file photo a U.S. paratrooper of the 82nd Airborne Division from Fort Bragg in North Carolina takes part in a military exercise 'Saber Strike 2018' at the at the Gaiziunai Training Area some 130 kms (80 miles) west of the capital Vilnius, Lithuania. The American military says it’s preparing a massive exercise early next year in Europe involving 20,000 soldiers from the U.S., the largest deployment across the Atlantic in more than 25 years for training. (AP Photo/Mindaugas Kulbis, file)