As Russia and Belarus prepare to commence their largest joint military exercise in three years, NATO defense officials are watching with “calm and confidence,” despite concerns about the projected massive size of the operation, and its potential to lead toward further Russian advances in the region.

The last Zapad (which translates to “West”) exercises Russia conducted in Belarus in 2013 were announced ahead of time as a joint military drill that would see as many as 13,000 military participants from the two nations.  Any more than 13,000 participants would have forced the Russian military to grant third-party observers the opportunity to oversee the drills.  The drills, however, quickly ballooned to over 90,000 participants, according to satellite imagery taken at the time, and served as a precursor to the Russian military annexation of Crimea.

This year’s Zapad drills, slated to begin on Thursday, came with similar announcements of only 12,700 participants overall, but NATO officials have remained steadfast in their predictions of upwards of 100,000 participants from the two nations.  These drills are a part of a rolling four-year cycle, with each year focusing on a different region (North, South, East, West), though the Zapad, or West, drills are often the subject of heightened tensions, as both Russia and NATO stare one another down on Europe’s Eastern border.

“NATO remains calm and vigilant,” NATO Secretary-General Jens Stoltenberg told reporters about Zapad last week.  NATO has been bolstering defenses throughout the region over the past year, including a sizable U.S. force in the Baltics, intended to serve as a response to any Russian effort to cross over the border into NATO territory.  Another Cold War era agreement prevents the United States from permanently manning the border itself.  There are currently some 4,000 NATO troops, many American, stationed in the Baltic states and Poland with the sole purpose of keeping an eye on their Russian neighbors.

Of course, not every NATO leader is toeing the Stoltenberg line, as Lithuanian Defense Minister Raimundas Karoblis pointed out, “We can’t be totally calm. There is a large foreign army massed next to Lithuanian territory.”

Last month, Gen. Ben Hodges, commander of U.S. Army forces in Europe, explained that some fear the Zapad drills could be a sort of “Trojan Horse,” allowing Russia to move heavy military equipment into Belarus, to then leave behind for use at a later date.  Although some fear these drills could create enough tension to ignite actual fighting between NATO and Russia, that ultimately seems far less likely than Russia simply using this event to better position military assets for later endeavors.

Despite the thousands of NATO troops in the region, it is considered unlikely that they would actually be able to truly stop, or even effectively slow, a massive Russian incursion into the Baltics, but if Russia had to move troops and assets into Belarus and Kaliningrad prior to the advance, satellite imagery could tip NATO officials off and give them the opportunity to begin their own logistical efforts to put personnel and assets in place to counter the Russian force.  If, instead, Russia leaves military assets in Belarus, they could potentially conduct a far swifter advance, with less NATO resistance along the way.

Concerns about how rapidly Russia could advance into NATO territory before being halted were highlighted earlier this year during a large-scale defense drill on the border between Poland and Lithuania.  A narrow stretch of land called the Suwalki Gap runs for about 65 miles through farmland, woods, and low hills of the region along the border of Poland and Lithuania.  What makes this tiny region so important to NATO officials isn’t the area it covers, however, but its neighbors.  It is flanked on either side by the Russian territory of Kaliningrad and Russian ally Belarus, and if captured quickly, could completely cut Estonia, Latvia, and Lithuania off from NATO allies to the West.