At 05:45 on May 23, 1944, 1,500 Allied artillery pieces commenced bombardment. Forty minutes later the guns paused as attacks were made by close air support and then resumed as the infantry and armour moved forward. The first day's fighting was intense: 1st Armored Division lost 100 tanks and 3rd Infantry Division suffered 955 casualties, the highest single day figure for any U.S. division during World War II. The Germans suffered too, with 362nd Infantry Division estimated to have lost 50% of its fighting strength.
Mackensen had been convinced that the Allies' main thrust would be up the Via Anziate, and the ferocity of the British feint on May 23 and May 24 did nothing to persuade him otherwise. Kesselring, however, was convinced that the Allies' intentions were to gain Route 6 and ordered the Hermann Göring Panzer Division, resting 150 miles (240 km) away at Livorno, to Valmontone to hold open Route 6 for the Tenth Army, which was retreating up this road from Cassino.
In the afternoon of May 25 Cisterna finally fell to 3rd Division who had had to go house to house winkling out the German 362nd Infantry which had refused to withdraw and, as a consequence, had virtually ceased to exist by the end of the day. By the end of May 25, 3rd Infantry were heading into the Velletri gap near Cori, and elements of 1st Armored had reached within 3 miles (4.8 km) of Valmontone and were in contact with units of the Herman Göring Division which were just starting to arrive from Leghorn. Although VI Corps had suffered over 3,300 casualties in the three days fighting, Operation Buffalo was going to plan, and Truscott was confident that a concerted attack by 1st Armored and 3rd Infantry Divisions the next day would get his troops astride Route 6.
On the evening of May 25 Truscott received new orders from Clark via his Operations Officer, Brigadier General Don Brand. These were, in effect, to implement Operation Turtle and turn the main line of attack ninety degrees to the left. Most importantly, although the attack towards Valmontone and Route 6 would continue, 1st Armored were to withdraw to prepare to exploit the planned breakthrough along the new line of attack leaving 3rd Division to continue towards Valmontone with 1st Special Service Force in support. Clark informed Alexander of these developments late in the morning of May 26 by which time the change of orders was a fait accompli.
At the time, Truscott was shocked, writing later:
I was dumbfounded. This was no time to drive to the north-west where the enemy was still strong; we should pour our maximum power into the Valmontone Gap to insure the destruction of the retreating German Army. I would not comply with the order without first talking to General Clark in person.... [However] he was not on the beachhead and could not be reached even by radio.... Such was the order that turned the main effort of the beachhead forces from the Valmontone Gap and prevented destruction of the German Tenth Army. On the 26th the order was put into effect. There has never been any doubt in my mind that had General Clark held loyally to General Alexander's instructions, had he not changed the direction of my attack to the north-west on May 26, the strategic objectives of Anzio would have been accomplished in full. To be first in Rome was a poor compensation for this lost opportunity...
On May 26, whilst VI Corps was initiating its difficult maneuver, Kesselring threw elements of 4 divisions into the Velletri gap to stall the advance on Route 6. For four days they slugged it out against 3rd Division until finally withdrawing on May 30, having kept Route 6 open and allowed 7 divisions from 10th Army to withdraw and head north of Rome...