The United States will be closely monitoring an upcoming major Russian military exercise this summer, says Secretary of Defense James Mattis. This includes the possibility of repositioning key military hardware to the area as a countermeasure to the Russians—if requested by NATO allies.

The Russian exercise, called “Zapad,” occurs every four years and will reportedly include up to 100,000 Russian soldiers. It has its roots in the former Soviet Union, where Soviet forces drilled under the pretense of a fictional invasion of Belarus by NATO forces. The training event was also a chance to showcase new military hardware and test those systems for the world to see. Following the collapse of the Soviet Union, the Russian military was unable to mount a similar training exercise until 2013, when it displayed the reinvigorated might of the Russian military that would soon see action in Crimea, Ukraine, and Syria.

This year’s version of Zapad, to commence in August and September, will once again occur in Belarus and the Russian enclave of Kaliningrad, prompting leadership in the Baltic nations to request more NATO military assistance in the face of tens of thousands of Russians drilling close to their borders. The Baltic states live more or less under continuous fear of a Russian incursion or other subversive activities initiated by the Kremlin to increase Russian influence in their countries, particularly after Russia’s successful annexation of Crimea and less-than-covert actions in Ukraine brought sanctions and international condemnation, but little else.

“We see that risks are increasing, and we are worried about the upcoming ‘Zapad 2017’ exercise, which will deploy a very large and aggressive force (on our borders) that will very demonstrably be preparing for a war with the West,” said Lithuanian President Dalia Grybauskaite. “This means that we will be talking with NATO about creating additional standing defense plans, about stationing additional military means, and about creating a faster decision-making process,” she said.