Eleven NATO countries have sent 4,000 troops to Lithuania, the largest Baltic nation, to participate in this year’s Iron Sword exercises. The war games are meant to test the country’s ability to rapidly deploy a large number of troops.

The exercise, which started on Sunday and is set to last till December 2, involves training at two separate sites in Lithuania.

“This time poses new unexpected challenges before our military. We have to prepare units and their commanders to efficiently respond to conventional military threats," General Waldemar Rupšys, the head of Lithuania’s Land Forces, told journalists ahead of the exercise.

This year’s Iron Sword maneuver, which is the third and, by far, largest held so far, involves almost 4,000 troops from the US, the UK, Germany, Canada, Poland, Romania, Slovenia, Luxemburg, and the three Baltic states. The exercises held in the last two years had 2,500 and just over 2,000 troops participating, respectively.

The troops will train to execute offensive and defensive operations, rapid deployments, and other tasks, Lithuania’s Defense Ministry said.

Iron Sword 2016 is Lithuania’s first chance to test its new Žemaitija (Iron Wolf) brigade, which was formed earlier this year. It currently has two battalions and support units, but is to add two more battalions next year. The brigade consists of soldiers conscripted after Lithuania reinstated mandatory military service in March of 2015.

NATO is placing additional military assets in Eastern Europe and conducting intensified training there, claiming that such measures are necessary to deter what it calls “Russian aggression.” However, Moscow denies threatening its neighbors and says the alliance is using the notion as a pretext to justify increased military spending and encroach on Russia’s border.

Lithuania, like some other European NATO members, is struggling to meet its obligation to spend two percent of its GDP on defense, but the government says it will be able to meet this benchmark by 2018.