"The final battle of Monte Cassino would be launched months later, known as Operation Diadem. The lessons of rushed attacks and poor weather had finally convinced the Allies that nothing short of a large-scale, coordinated offensive across a wide front would break the Gustav Line. In order to accomplish this, more divisions were moved up, including American, Polish, and Indian forces along with a division of Canadian tanks. The Allies prepared this offensive over the span of two months, with small troop numbers moving and building along the front to avoid arousing German suspicion. This, along with fabricated training and communications, suggested to the Germans that a second naval landing would occur north of Rome, drawing away their reserves.
Finally, after months of failed offensives, the Allies conducted a final, massive push on May 11th, reaching all the way from the mountains of the Rapido River to the coast. A number of factors helped the Allies in this fourth attempt, including far better weather and ground conditions, which allowed their troops to more easily advance. Furthermore, a large detachment of French colonial mountain troops was able to cross a section of the undefended mountain, believed to be impassable by the Germans, allowing them to threaten German supply lines and the flanks.
Finally, by holding a bridgehead at the town of Cassino long enough, Canadian armor was able to push over the Rapido River and exploit the beachhead made by the infantry. Ultimately, it would be the Polish who reached the abbey first through brutal up-hill fighting, to the point where only a few men were left fit enough to climb the last stretch into the abandoned monastery to raise the Polish flag on May 17th."