The Gathering Storm

Approximately 136,000 Russian troops have moved near the Ukrainian border. The troops, including more than 30,000 in Belarus, have begun building staging grounds and field hospitals, prompting NATO to suspect future military action.

Some military strategists have predicted that the forces in Belarus are preparing for a push on the Ukrainian Capital, Kyiv. To make matters worse, Russian separatists in Crimea have participated in attacks against Ukrainian forces, wounding three soldiers.

US Response 

In response to these growing tensions, the US moved parts of the 1st Brigade, 82nd Airborne Division, and the 2nd Cavalry Regiment to nearby Romania and Poland as a precaution.
However, Russia recently condemned these actions. Russia insists that it is only taking part in joint military exercises with Belarus. Russia and Belarus paint Ukraine as the aggressor.
Alexander Lukashenko, the president of Belarus, said the exercises are meant to secure the border with Ukraine. Belarus claims that Ukraine has been sending drones into its airspace.
Some strategists believe that Russian President Vladimir Putin is merely trying to annex Ukraine by the threat of force, rather than by force itself. One reason why newly deployed American units are mostly light is that they are not expecting contact with Russian forces. The other reason is that these forces are either air-transportable or already in Europe.
Other strategists believe that invasion is a very real threat that will not take long to unfold.

Worst Case Scenario 

If Russian aggression spills over into other Eastern European nations, the US forces in Europe won’t be able to hold back the mostly mechanized Russian invasion force.

Russia is also adamantly against Ukraine joining either the European Union or NATO. Russian Deputy Foreign Minister Sergei Ryabkov stated, “For us it’s absolutely mandatory to ensure Ukraine never, ever becomes a member of NATO.”

Commander’s Intent

Russia most likely wants Ukraine to serve as a buffer zone between itself and NATO, much like how China uses North Korea as a buffer zone against the US-backed South Korea. They want the United States to withdraw forces from Eastern Europe, which are now roughly equivalent to two divisions.

Russia also fears that a NATO-affiliated Ukraine would retake Crimea, which Russia annexed in 2014. President Putin has produced a blog post explaining why Russians and Ukrainians are one people. The Russian government most likely views Ukrainian opposition as a form of civil war.