[pierrem15]

The part of Czechoslovakia taken in 1938 had been under dispute since 1918 and the Poles only occupied it to prevent the Germans from moving in when they took over the rest of Czechoslovakia in 1938.

In 1968, the Polish army was forced by the Russians to participate in the invasion. Polish tanks crossed the border, but with their guns turned backwards as a sign of protest.

There are long standing ties between the Western part of Ukraine and Poland (not always happy ones), but a lot of Western Ukrainians have seen the political and economic development of Poland since 1989 and want the same thing.

There is no way in hell that Poland will sit back and do nothing as Ukraine becomes a Russian satellite or even worse is re-annexed by the Russian Empire, and if the Russians have a 'right' to intervene for 'historical' reasons, why not the Poles?

[Cronos]

Correct. Poland needs buffer states between it and the Russian eagle. A buffer state is always a good idea. The loss of the Polish Lithuanian commonwealth in 1791 directly lead to ww1 as it put Germany and Austria-Hungary in direct contact with each other.