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 in more than 25 years for training.

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."