/ Has Poland ever participated in any big wars (except WW2)?
Czechs invaded polish territory twice, in 1919 when Poland was fighting Ukraine, Russia and Germany
Actually Czech forces requested that Poland observed the terms of the interim agreement which Poland had signed. And Poland was fighting neither Russia nor Germany at that time.
Funny how your version of history somehow omits the two Polish invasions of Czechoslovakia in 1918.
suddenly when all of her armed forces are elsewhere Poland finds itself assaulted without cause or provocation
No cause other than Poland deciding to exercise sovereign rule in disputed territory which it had agreed it would not exercise sovereign rule in. And it was most certainly not sudden: Poland was given more than enough notice with regard to what would happen if she did not keep to her word.
it was unreasonable of Czechs to expect no retaliation
You mean that it was unreasonable of them to expect Poland to keep her word? Well, on that we would agree, given the number of treaties which Poland broke in the years 1918 to 1939.