Currently, NATO’s involvement in Ukraine is verbal and structural support (see this Factsheet on NATO-Ukraine relations before the crisis and information on the NUC, or NATO-Ukraine Commission). NATO could take a more direct role in the conflict as it has in other armed conflicts and disputes over sovereignty/independence.
One way to imagine what NATO’s actions and the results of those actions would be is to reason by analogy. This means drawing on a similar situation by illustrating how the two are similar, and therefore you can draw conclusions about the Ukraine from empirical evidence.
One analogy could be to a past NATO peacekeeping operation. Look to the Current Operations and Missions( perhaps NATO in Kosovo and the conflict between Sovereignty vs. self-rule) or Terminated Operations and Missions (perhaps NATO in Bosnia and Herzegovina)
You can also look to other current civil wars and armed conflict where foreign actors have intervened.
Don’t forget other peacekeeping bodies, such as the UN, could have acted in a similar situation and provide and analogous scenario.
While singular instances are helpful to draw analogies, you want to back up your claims with Meta-analysis, or research/data that looks at a broad spectrum of NATO or peacekeeping activities and their success or failures. Here are some examples of research and scholarship that would be helpful to this point.
Dr David BAKRADZE 2001
Megan Gleason-Roberts and Alischa Kugel
William J. Durch, US Institute of Peace Press, 2006
Anand Menon, Jennifer Welsh 2011