The Poles

The Polish contribution to the fighting was the II Polish Corps (Lt Gen Wladyslaw Anders) composed of 3 divisions: 3rd Carpathian Infantry (Maj Gen Bronislaw Duch), 4th Kresowa Infantry (Maj Gen Nikodem Sulik) and 2nd Armoured (Maj Gen Bronislaw Rakowski).  Of all the Allied combatants the Poles were in the unique position where, being cut off from their occupied country, they had very limited access to replacements that could replace the heavy casualties the upcoming fight was expected to cause.  However, they chose to take part in the battle as they felt that it would reinforce the political position of the Polish government-in-exile in London in the political battle for their country's future.  This was of utmost importance as it was at that time the Soviets were pushing into Poland, bringing with them a Soviet sponsored Communist government that would have made Poland a Soviet satellite state. 
The Corps was deployed in the mountains above Cassino with the objective of taking the monastery and cutting across the peaks into the Liri Valley.  This move was to threaten the encirclement and thus force a withdrawal of the Cassino position in order to open the road to Rome.  The first Polish attack went in during a powerful artillery barrage and their left units took the key Calvary Mount before the Germans could return to their fighting positions from the bunkers where they waited out the artillery barrage.  However under constant fire from surrounding heights and with ammunition running out they are pushed back by the 5th German counterattack that same day.  Limited progress was also made on their right.  The second Polish attack was launched using all available manpower (including clerks, cooks, drivers, etc.) to replace the heavy losses from the first attack and with the support of tanks that managed to bypass German minefields by climbing seemingly unpassable slopes.   They take heavy losses but also manage to decimate the defenders and make slow progress forward.  By the time the Germans withdraw from the Gustav line on the 17th, the Poles are close to a breakthrough.  However, as the sun rises on the 18th, the Germans are already gone and a Polish patrol enters the monastery to raise their flag over the ruins.