Russia is calling their Zapad-2017 operation a military exercise but Nato is on edge as up to 100,000 troops conduct war games in Belarus and the small Russian enclave of Kaliningrad.
Lithuanians are particularly anxious on the border with Russia in the north along the Baltic Sea, while the operation takes place close to Poland.
The exercise is to be held between September 14-20 and comes against a backdrop of tension between Russia and the US and EU.
FBI agents are already investigating possible interference by Moscow in the 2016 elections.
Russian President Vladimir Putin: Source: Twitter
Russian troops have already begun pouring into Belarus, according to updates on social media, saying there's nothing unusual to its troop build-up which it conducts every four years.
A Nato spokesperson said they will "closely monitor exercise Zapad 17 but we are not planning any large exercises during Zapad 17. Our exercises are planned long in advance and are not related to the Russian exercise.”
Military analysts say the speed at which Russian troops are withdrawn at the end of the exercise will be an indicator about whether Moscow is planning further action.
Ukrainian officials in Kiev, who are in a civil war battle against insurgents in the east of the country, claim it is part of Russia's move towards a full-scale invasion of western Ukraine.
But the Zapad exercise has been held every four years since 1999 and is part of an annual rotating series of exercises that Moscow uses as part of its training. These rotate through the various commands – eastern, central and western Caucasus – and has been part of the blueprint for training for years.
The size of the war games has increased tension, with Moscow refusing to confirm the exact number of troops likely to be mobilised, but Nato has said it expects 100,000.
Posters in the Belarus City of Minsk read "Russians Go Home".
Previous Zapad exercises, for example Zapad-2013, involved around 75,000 troops and personnel. According to the Russian defence minister, the focus is to be on "joint planning, command tactics and joint troop formations".
In June 2017, Nato Secretary-General Jens Stoltenberg said the exercise was part of Russia's routine.
"We are going to follow and monitor the Zapad exercise area here closely, and all nations have the right to exercise their forces but it is important that nations, be it Belarus or Russia, that they do that in accordance with well-established guidelines and agreements and international obligations, and we have something called the Vienna document, which outlines how exercises have to be notified and be subject to international inspections. We call on Russia and also Belarus to do that in accordance with the Vienna document so that we have transparency, predictability related to Zapad 2017," he said.
"We are also working in the framework of the Nato-Russia council to have more transparency, predictability, connected to military posture but also exercises, and that is always important, but especially important now when we see more of a military presence along our borders in this region. It’s even more important to have transparency [within] international observation of exercises like Zapad."
Belarus is a member of Collective Security Treaty Organisation (CSTO), which is a military alliance of some of the former USSR republics led by Russia.
The plan of the exercise was approved by Belarus President Alexander Lukashenko on March 20 2017, who at the time envisaged two stages and its theme was defined as "the use of groupings of troops in the interests of ensuring the military security of the Union States".
While the Vienna document of 2011 stipulates that at no time may Russian troops and personnel number more than 13,000 on the ground in Belarus, the military action across a broad front means Moscow could use Belarus as a conduit for larger numbers to move into Kaliningrad and even the Kola Peninsula within Russia's Arctic Circle.