Air defense was a big part of the exercise:
Reported use of strategic down to tactical air defense, combined with the employment of additional assets, in the very early stage of Zapad 2017 confirms that the Russian military rehearsed a conventional response to a massive aerial attack. During the first phase, joint military forces worked on raising combat readiness among the deployed force groupings, moving troops, deploying command-and-control assets, as well as organizing interactions among these forces and affording force protection. Reportedly, aviation and air-defense units from the 6th Army Air Force and Air Defense in the Western MD conducted various tactical episodes aimed at repelling “massive air strikes” by a conventional opponent.
This kind of exercise could have been done anywhere if it was simply to endure a NATO air assault.
I think Belarus as the location for Zapad 2017 was precisely for the purpose of practicing an invasion of Belarus. That would be done either for absorbing and annexing Belarus or in stride to invade Lithuania and Poland.
If Russia moves into Belarus to take it over, Belarus may or may not cooperate and oppose NATO air operations with their own air defenses. So if NATO intervenes, Russia will need to protect their army and national guard units moving in to take over.
If there is one thing that Russia has learned from American interventions since Kosovo, it is that American-led Western air power is awesome. And if the Western air power is allowed to work over your army, your army can be crippled and pinned in place to avoid destruction.
So the major requirement for a Russian invasion is not the ground forces to defeat the Belarusian military or even to defeat NATO ground forces (which could not intervene in force rapidly), but the mobile air defenses to hold off American-led air power to cripple the Russian army before it can settle into defensive lines in Belarus or into offensive positions prior to advancing west.
NATO would need to use surface-to-surface weapons and stealth bombers to take out the early air defenses so that the rest of NATO's air power can hit the road and rail net and airfields plus the moving air defense units before they set up to leave the rest of the Russian military and other infrastructure needed to use Belarus vulnerable to non-stealthy aircraft.
I have noted that Belarus is perhaps the most important territory in Europe today.
