It looks like the Russian Zapad 2017 exercise carried out last month practiced an offensive through Belarus to link up with their Kaliningrad exclave by blasting through Lithuania and Poland.
Do read it all. Given the Russian skill in lying, the exercise starts with ludicrously small NATO attacks on Russia which Russia responds to with a large offensive after easily fending off the attacks.
And note that the Russians practiced using nukes at the end of the successful offensive, perhaps to deter a NATO counter-attack. Which admits the weakness of the Russian military despite some pockets of excellence and the ability to overwhelm a weaker foe by seizing the initiative with local superiority.
Note too that the parameters of the offensive leave NATO forces further north in Estonia and Latvia cut off in a pocket relying on tenuous sea and air links. Which is why I am against anything more than a NATO tripwire force to bolster local forces in the Baltic states.
Russia is weaker than America and weaker than NATO in conventional forces. No doubt. But the situation is modified by geography.
As I noted long ago, the weaker Soviet Union could put a lot of force into the fight in NATO's central front rapidly while much of NATO's firepower had to rush to the battlefield. If the USSR could reach the Rhine before the cavalry arrived, the Soviets would win the campaign. And perhaps the war.
Russia, while weaker than the Soviet Union, faces dramatically weaker foes on the front line in the Baltics. And NATO reinforcements are fewer and farther away than when the Fulda Gap was the front line.
So yes, Russia could achieve their objectives in the initial campaign. NATO will have to mobilize and counter-attack to free what Russia conquers.
And given that Russia practiced moving a mobile air defense shield into Belarus to cover the movement of their army to hit Poland and Lithuania, we are reminded of the importance of a neutral Belarus, which may be the most important territory in Europe today.
UPDATE: What are the limits of Belarus's sovereignty?
NATO has an interest in defending Belarus sovereignty notwithstanding its authoritarian government.