Extending Russia, Rand Corporation 2019

The United States could also become more vocal in its support for NATO membership for Ukraine... While NATO’s requirement for unanimity makes it unlikely that Ukraine could gain membership in the foreseeable future, Washington’s pushing this possibility could boost Ukrainian resolve while leading Russia to redouble its efforts to forestall such a development.

Expanding U.S. assistance to Ukraine, including lethal military assistance, would likely increase the costs to Russia, in both blood and treasure, of holding the Donbass region. More Russian aid to the separatists and an additional Russian troop presence would likely be required, leading to larger expenditures, equipment losses, and Russian casualties. The latter could become quite controversial at home, as it did when the Soviets invaded Afghanistan.

https://www.rand.org/content/dam/rand/pubs/research_reports/RR3000/RR3063/RAND_RR3063.pdf