What Happened Before Avengers Disassembled?

What Happened Before Avengers Disassembled? A couple notes here: 1) The events of the Iron Man and Captain America trades begin before Avengers Disassembled, but run concurrent with the major event eventually. I read the individual tie-ins after reading Avengers Disassembled as a whole and enjoyed them fine that way. What led to Avengers Disassembled?