What were the major factors that allowed the British to win the Battle of El Alamein?
El Alamein is considered one of the more decisive battles in the war for North Africa during World War II. A major factor overall in that war (and one we have seen again here in the recent civil war in Libya) is that logistics (supply lines) are very long and difficult to maintain.
El Alamein was a defensive line created by the British between the Qattara Depression and the Mediterranean Sea, a last ditch line designed to prevent German Field Marshall Erwin Rommel's advance on Cairo, Egypt and the Suez Canal. By the time German forces approached the line, Rommel was badly overextended. After capturing Tobruk, his troops needed rest and time to resupply, but Rommel drove them west, preferring not to give the British time to regroup and organize.
Ultimately, the German Afrika Korps was too exhausted to break the line, and Rommel ordered a halt to re-equip and resupply, hoping to finish Montgomery and the British at a later time.
What most people forget was that El Alamein was two battles, the first in Summer 1942 that halted the German advance, and another in late 1942 where the British broke the German lines and began the long retreat back across Libya and, ultimately, out of Africa altogether. El Alamein was the turning point.