On Thursday, Russia and Belarus began a week of war games, which rattled Eastern European NATO states and led to rampant speculation about the true purpose of the exercise before they even began. Dubbed “Zapad 2017” (meaning “West”), the games are meant to simulate an attack by a coalition of fictional Western states — Lubenia, Vesbaria, and Veishnoria — which some observers see as stand-ins for Poland, Lithuania, and Latvia.

The Zapad exercises are neither new nor unusual: The Soviet Union and Warsaw Pact countries carried out a series of “Zapad” exercises simulating NATO invasions from the West throughout the ’70s and ’80s, and this is the fourth conducted by the Russian Federation since 1999. What has Europe worried is that Zapad 2017 comes at a time of considerable tension in Eastern Europe, while the memory of 2013, when Russia used the Zapad exercise to lay the groundwork for the seizure of Crimea from Ukraine the following year, remains fresh. Military exercises were also used as a disguise for the invasion of Georgia in 2008.

By and large, NATO does not see Russia pulling a similar stunt with this year’s exercise, but the alliance has expressed concern that Moscow is not being transparent about the number of troops involved in the exercise, claiming that under 13,000 soldiers are participating when the total could really be as high as 100,000. What the Russian and Belarusian governments have depicted as a routine defensive drill looks from the outside like a rehearsal for a major war.