The simple answer is: With their military.

Here the much more complex answer:

You can´t compare the reaction of the EU towards the war in Ukraine (a non-ally) with the reaction of the EU if they themselves would have been attacked. There are plans and procedures available that would come into effect in case of war that would cut a lot of the red tape that currently hinders the support of Ukraine. The military would take over, Europe would change into a war economy and start pushing against the invasion forces of Russia. The Baltics would be - most likely in a bad state as any NATO forces stationed there would keep an eye on not being cut off mainland Europe. But Poland alone would have been the first major block for Russian forces. More and more forces would join the battle while all over Europe people would get drafted into arms to join the party. With time all Russian forces would have been neutralized and pushed out of Europe proper.

Even when the US would have left NATO this would have happened. It would have caused more casualties and delayed the date of defeat of Russia but it would not change much until the moment we get to nuclear weapons. But this is a whole different story as conventional armies don´t add anything to those kind of equations.

Because I totally agree, Europe has abandoned its military forces but Russia threw their military out of the window and shot it after it crash landed.