I understand that the atomic bombs on Hiroshima and Nagasaki brought Japan to its knees. But I've always been kind of hazy on what exactly brought Germany to its knees.
It was more a series of straws, the very last being the failure to protect Berlin itself from Soviet forces, though by then there was little question about whether Germany was already effectively defeated.
The first major straw was the German defeat in the battle of Stalingrad in early 1943, after which the Germans began consistently being defeated and retreating on the Eastern Front. The next major straw was probably the Normandy invasion in mid 1944, or at least the failure of Germany to repulse it.
The actual big "last straw" was probably the failure of the Battle of the Bulge in January of 1945. The Germans had collected all their good equipment and troops together and made an attempt to push the Allies back and to take control of Antwerp, which would be a significant logistical setback. Even this was only an attempt to work toward a negotiated surrender, at this point it was already fairly clear the Germans were not going to win the war, and at best they could preserve something in their loss.
After the German breakout attempt was pushed back the writing was on the wall. Indeed, at the Yalta conference in early February 1945 the major Allied powers decided among themselves how they were going to deal with Germany after the war. In mid-1944 the front-line in the East was roughly on the borders of the USSR, within 6 months the Soviet forces would push that line across nearly the entirety of Eastern Europe, right to the border of Germany. In weeks the Soviets managed to push deep into German and Austrian territory, reaching Vienna in March 1945, and Berlin in April. The battle of Berlin took about 2 weeks and resulted in about half a million soldiers killed or wounded between both sides.
There was a last ditch effort to defend against the Soviets in the Battle of Berlin which failed to come to fruition, it was the news of this failure (on April 22) and Hitler's reaction to it that make up the famous scene from the movie "Downfall". At that point there was very little hope of Germany being able to halt the Soviet advance.
There were still German forces in the field and still lots of unconquered territory held by Germany, but because of the presence of the high-command in Berlin and the suicide of Hitler and other top officials that was the effective end. It was only about a week and a half between Hitler's suicide and the surrender of all German forces.