Decision to stop American and British advance to Berlin:

Why American and British armies halted their advance at the Elbe River in World War 2 and allowed the Russians to take Berlin.
Following the crossing of the Rhine River in March 1945, the American and British armies moved very rapidly east of the river. There was intermittent fierce opposition from the German armies to stem the advance, but the Allied movement was relentless. The second armored division and elements of the thirtieth infantry division reached Magdeburg on or about April 11, 1945 and proceeded to cross the river in boats. Infantry dug in on the east bank of the river and were forced to remain in position due to fierce artillery shelling from German units. Engineers were almost successful in putting up a bridge from Magdeburg, but the Germans knocked it out. At that time, supreme headquarters opposed further advance to Berlin. Field Marshal Bernard Montgomery and Winston Churchill were in favor of moving ahead, but General Omar Bradley opposed the idea and prevailed. Abandoned on the east bank of the river, some men were able to get back across; others were killed and a number of others became prisoners of war.

The following links will provide some background to illuminate this final action, which the Germans in their daily communiqué labeled as "The Little Dunkirk".

http://www.bartleby.com/65/ya/YaltaCon.html

http://www.spartacus.schoolnet.co.uk/2WWyalta.htm

http://www.army.mil/cmh-pg/books/70-7_22.htm