where not only US troops were involved but French, Polish and British
Destruction of the Abbey
Increasingly, the opinions of certain Allied officers were fixed on the great abbey of Monte Cassino: in their view it became the abbey - and its presumed use as a German artillery observation point - that prevented the breach of the ‘Gustav Line,’ and not the futility of successive, costly frontal attacks on determined and skilled defenders.
The British press and C. L. Sulzberger of the New York Times frequently and convincingly and in (often manufactured) detail wrote of German observation posts, artillery positions, etc. inside the abbey. Air commander General Ira C. Eaker personally observed during a fly-over German uniforms on clothes lines inside the walls, and German soldiers within 50 feet of the abbey walls.
The view in New Zealand Corps H.Q., as articulated in the writings of Major-General Howard Kippenberger, commander of 2 New Zealand Division, was that the monastery was probably being used as the German's main vantage point for artillery spotting since it was so perfectly situated for the purpose that no army could refrain from using it. There is no clear evidence to this effect, but he went on to write that from a military point of view the current state of occupancy of the monastery was immaterial: "If not occupied today, it might be tomorrow and it did not appear it would be difficult for the enemy to bring reserves into it during an attack or for troops to take shelter there if driven from positions outside. It was impossible to ask troops to storm a hill surmounted by an intact building such as this, capable of sheltering several hundred infantry in perfect security from shellfire and ready at the critical moment to emerge and counter-attack....Undamaged it was a perfect shelter but with its narrow windows and level profiles an unsatisfactory fighting position. Smashed by bombing it was a jagged heap of broken masonry and debris open to effective fire from guns, mortars and strafing planes as well as being a death trap if bombed again. On the whole I thought it would be more useful to the Germans if we left it unbombed".
Major General Francis Tuker, whose division, 4th Indian Division, would have the task of attacking Monastery Hill, had made his own appreciation of the situation. In the absence of detailed intelligence at U.S. 5th Army H.Q., he had found a book dated 1879 in a Naples bookshop giving details of the construction of the abbey. In his memorandum to General Freyberg he concluded that whether the monastery was currently occupied by the Germans or not, it should be demolished to prevent its effective occupation. He also pointed out that with 150 foot high walls made of masonry at least 10 feet thick there was no practical means for field engineers to deal with the place and that bombing with "blockbuster" bombs would be the only solution since 1000lb bombs would be "next to useless".
On February 11, 1944, the acting commander of 4th Indian Division, Brigadier H. W. Dimoline requested the bombing of the abbey of Monte Cassino. Major General Tuker, reiterated again his case for bombing the monastery from his hospital bed in Caserta, where he was suffering a severe attack of a recurrent tropical fever. Gen. Bernard Freyberg transmitted his request on February 12, 1944. Lt. Gen. Mark W. Clark of Fifth Army and his chief of staff Major General Albert Gruenther were unconvinced of the “military necessity”. When handing over the U.S. II Corps position to the New Zealand Corps, General Butler, deputy commander of U.S. 34th Division had said "I don't know but I don't believe the enemy is in the convent. All the fire has been from the slopes of the hill below the wall". Clark pinned down theater commander General Sir Harold Alexander: “You give me a direct order and we’ll do it.” He did. The ancient abbey’s fate was sealed.
The bombing mission in the morning of 15 February 1944 involved 239 heavy (B-17) and medium (B-26) bombers, in all they dropped 453 tons of ordnance on the Monte Cassino complex within four hours, followed the next day (16 February 1944) by concentrated artillery barrages and additional tonnage onto the ruins by 59 fighter bombers.
The air raid however, had not been co-ordinated between the air and ground commands, with the timing driven by Air Force projecting it as a separate operation, considering the weather and to be fitted in with other requirements on other fronts and theaters and without reference to the ground forces (indeed, the Indian troops on the Snake's Head were taken by complete surprise when the bombing actually started). The raid took place two days before The New Zealand Corps were ready to launch their main assault. Many of the troops had only taken over their positions from U.S. II Corps on February 13 and preparations had been held up by difficulties in supplying the newly installed troops with sufficient material for a full-scale assault because of incessantly foul weather, flooding and waterlogged ground. In the context of the planned assault the bombing therefore achieved nothing and it helped no-one.
After the bombing
Pope Pius XII was silent after the bombing, however, his secretary of state, Cardinal Maglione bluntly stated to the senior U.S. diplomat to the Vatican, Harold Tittmann, that the bombing was “a colossal blunder . . . a piece of gross stupidity.”
What is certain from every investigation that followed since the event, is the fact that the only people killed in the monatery by the bombing were Italian civilians seeking refuge in the abbey. There was never any evidence, then or now, that the bombs dropped on the Monte Cassino monastery that day killed a single German. However, given the imprecision of bombing in those days (it was estimated that only 10% of the bombs from the heavy bombers, bombing from high altitude, hit the monastery) bombs did fall elsewhere and killed German and Allied troops alike.
The American government's position on the bombing changed over a quarter century in installments, in small corrections, in evasive and subtle rewording from “irrefutable evidence” of German use of the abbey to the final correction in 1969 of the U.S. Army’s official history, that “the abbey was actually unoccupied by German troops.”
On the day after the bombing at first light, most of the civilians still alive fled the ruins. Only about 40 people remained: the six monks who survived in the deep vaults of the abbey, their then 79 year old abbot, Gregorio Diamare, three tenant farmer families, orphaned or abandoned children, the badly wounded and the dying. After artillery barrages, renewed bombing and attacks on the ridge by 4th Indian Division, the monks decided to leave their ruined home with the others who could move at 7:30 in the morning on 17 February 1944. The old abbot was leading the group down the mule path towards the Liri Valley, reciting the rosary. After they arrived at a German first-aid station, some of the badly wounded who had been carried by the monks were taken away in a military ambulance. After meeting with German officers, the monks were driven to the monastery of San Anselmo. One monk, Carlomanno Pellagalli, returned to the abbey; when he was later seen wandering the ruins, the German paratroopers thought he was a ghost. After 3 April 1944 he was not seen anymore.
Paratroopers of German 1st Parachute Division occupied the ruins of the abbey and turned it into a fortress that withstood all that was thrown at them for the next three months.
http://en.wikipedia.org/wiki/Battle_of_Monte_Cassinosummary : the bombing of the Abbey was an ALLY SNAFU even if the US Air Force bear the direct responsability. Curiously the Abbey's treasures were saved by the Germans :
In the course of the battles the historic monastery of Monte Cassino, where St. Benedict first established the rule that ordered monasticism in the west, was entirely destroyed by the US Army Air Force. Fortunately, prior to the offensive, German Lieutenant Colonel Julius Schlegel has initiated a transfer of the library (consisting of approx. 1,200 historical documents and books) and further art treasures of the monastery to the Vatican in Rome in order to prevent them from being destroyed.
Immediately after the cessation of fighting at Monte Cassino, the Polish government in Exile (in London) created the Monte Cassino campaign cross to commemorate the Polish part in the capture of the strategic point. Later, an imposing Polish cemetery was laid out; this is prominently visible to anybody surveying the area from the restored monastery.
The British cemetery on the western outskirts of Cassino is also the final resting place of the New Zealanders, the Indians and the Gurkhas. The French and Italians are on Route 6 in the Liri Valley; the Americans are at Anzio. The Germans are on the eastern side of Cassino in the Rapido Valley.
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I must add that similar criminal idiocies were conducted in Normandy, the Netherlands and of course Germany.
Today it's called collateral damage.