Montgomery offered Eisenhower his suggestion for an aggressive assault against Germany. He proposed to Dwight Eisenhower a thrust into the Netherlands to control several key bridges in preparation for a subsequent armor assault across Germany's flat northern plains. Accepted by Eisenhower, Operation Market Garden was launched, but it met a complete failure with heavy loss of lives. Eisenhower, disappointed with the defeat, transferred Montgomery from the front lines to become the commander-in-chief of British occupation forces, but he stressed that Montgomery's skills were not to be doubted. "Those critics of Montgomery who assert that he sometimes failed to attain the maximum must at least admit that he never once sustained a major defeat", said Dwight Eisenhower. Addressing the criticism that Montgomery lacked aggressiveness, Eisenhower responded that "caution and timidity are not synonymous, just as boldness and rashness are not!" These words of Eisenhower's were rather generous considering Montgomery's attitude toward Eisenhower bordered on insubordination. During a one-on-one planning session for Market Garden between Montgomery and Eisenhower, Montgomery lectured Eisenhower as if Eisenhower was a child. Eisenhower waited until Montgomery paused for breath, and interrupted; "Steady Monty," Eisenhower said. "You cannot talk to me like this. I am your boss." Montgomery, his ego suddenly deflated, mumbled his apologies; "Sorry, Ike".
When the Germans embarked on the Ardennes offensive on 16 Dec 1944, known as the Battle of the Bulge to the Western Allies, the US 1st Army was split in two groups by the Germans. While Bradley maintained communications with the southern group, he lost touch with the northern group. Montgomery was the nearest Allied officer to the northern units of the US 1st Army, so he absorbed the American units into his command. German General Hasso von Manteuffel of the 5th Panzer Army praised his opponent's quick decision, noting that
<dir> Montgomery's contribution to restoring the situation was that he turned a series of isolated actions into a coherent battle fought according to a clear and definite plan. It was his refusal to engage in premature and piecemeal counter-attacks which enabled the Americans to gather their reserves and frustrate the German attempts to extend their breakthrough. </dir> As the Germans began to lose their initiative, Eisenhower ordered Montgomery to go on an offensive on 1 Jan 1945 in an attempt to envelope the German forces. On the grounds that his men were not prepared to march through a snowstorm, he delayed his attack for two days, by which point the bulk of the German forces escaped what could had been a pocket.
During the Allied advance to the Rhine River, Montgomery's careful planning directly led to the low casualty rates among his units. His 21st Army Group was ordered to swing north to take Hamburg, Germany and to seal the base of the Danish peninsula to block a potential Russian westward advance beyond Berlin. On 4 May 1945, in a tent in the region of Lüneburg Heath in Lower Saxony, Germany, he accepted the surrender of German forces in northern Germany, Denmark, and the Netherlands.