From Wikipedia

Himmler, Hitler And The End Of The Reich

By 1945, Soviet forces were approaching Berlin. American and British forces had taken most of western Germany and met their Soviet allies at Torgau on the Elbe on 26 April 1945 (Cohen). With Berlin under siege, Hitler and other key members of the Nazi regime were forced to live in the armoured underground Führerbunker while the upper terrain of Berlin was constantly shelled by the Red Army.

The US Army blows up the swastika atop the Nazi Party rally ground (Zeppelin field) in Nuremberg.

In the underground bunker Hitler grew increasingly isolated and detached from reality. Increasingly, he exhibited signs of mental illness. It was claimed that, at a meeting with military commanders, Hitler began to consider committing suicide should Germany fail to win the war. Berlin was eventually surrounded and outward communications between Berlin and the rest of Germany were cut off. Despite evident total defeat, Hitler refused to relinquish his power or surrender.

With no communications coming out of Berlin, Hermann Göring sent an ultimatum to Berlin that he would take over the Nazi regime in April if his ultimatum was not responded to, in which case Hitler would have been deemed to be incapacitated as leader. Upon receiving the message, Hitler angrily ordered Göring's immediate arrest, and had a plane deliver the message to Göring in Bavaria. Later, in northern Germany, Reichsführer-SS Heinrich Himmler began communicating with the western Allies about negotiating peace. Hitler once again reacted violently to Himmler's attempts to seek peace and ordered both his arrest and execution.

With no intent by Hitler to surrender, intense street fighting continued in the war-torn ruins of Berlin between remnant German army forces, Hitler Youth, and the Waffen-SS against the Red Army. This battle was known as the Battle of Berlin. The German forces by this time were severely depleted, large numbers of German children and the elderly were forced into conscription by the Nazis to fight against the Red Army in the remaining pockets of territory not controlled by the Red Army in Berlin.