Certainly any possibility of a direct US-Russian force-on-force engagement would be a major concern, as it could spark the beginning of a dangerous great-power conflict
If Russia invades Ukraine, US forces will not defend Ukraine. It is that simple and clear, according to Pentagon spokesman John Kirby, who told reporters that US moves to reassure NATO within Eastern Europe does not translate into any plans to have forces oppose Russia in Ukraine.
US Troops & Ukraine
“The president has made clear that U.S. troops are not going to be fighting in Ukraine. The troops that we have added to the already 80,000 that are based in Europe are going to reassure our allies and our partners to deter aggression against the alliance to conduct some joint training,” Kirby told reporters, according to a Pentagon transcript.
The red line for the US and NATO, when it comes to a willingness to engage in actual military conflict, seems to involve NATO and Eastern Europe. Therefore, US and European force maneuvering is likely intended to send a clear message that any movement Westward beyond Ukraine would not be tolerated. Combined US and NATO allied forces are conducting drills and solidifying interoperability in the event that they might need to enter combat.
This apparent distinction, however, does not mean the US will do nothing in response should Russia actually invade. While any actual response would be likely to change depending upon emerging circumstances, the US has already made its support for Ukraine very clear. The US is moving thousands of troops to Eastern Europe and deploying new forces from the US. The US has also sent Javelin anti-tank missiles to Ukraine, weapons Kirby said were initially sent as purely defensive weapons.
Some have speculated that a Russian invasion of Ukraine could result in a need to conduct a longer-term counterinsurgency campaign against Ukrainian forces potentially supported by the US and allied partners. Any counter-Russian insurgency in Ukraine is likely to be quite challenged, given that Russia operates as many as 850,000 active duty forces, according to Global Firepower, as compared to Ukraine’s 200,000