The joint Lithuanian, Polish and Ukrainian Brigade, LITPOLUKRBRIG is an example of a Western hybrid army in Eastern Europe. This is an element of aggressive hybrid war to destabilise Russia.
In reality, Poland and Lithuania do not plan to send armies to Donbas to support Kyiv. Instead, through LITPOLUKRBRIG, Poland and Lithuania intend to contain Russia by supplying Soviet weapons to Kyiv, amongst others. Military associations such as LITPOLUKRBRIG give the US the means to preserve its influence in Eastern Europe, even if the EU creates a common army.