From the New York Times:
The new strategy required more American forces, and the generals initially had different views as to how large the American troop reinforcement should be.
Gen. George W. Casey Jr., the top American commander in Iraq, and Gen. John P. Abizaid, the leader of the United States Central Command, who have long argued that sending too many troops would put off the day when the Iraqis would take responsibility for their own security, initially had a more modest approach. According to a senior administration official, they thought two additional American combat brigades would be sufficient for Baghdad. A third would be held in reserve in Kuwait and two more would be on call in the United States.
But Lt. Gen. David H. Petraeus, whom Mr. Bush has selected to replace General Casey, wanted to ensure that he had enough troops to carry out what by all accounts will be an extremely challenging mission. He sought a commitment that all five combat brigades would be sent.
Mr. Bush opted for the larger commitment. Five brigades are to be sent to improve security in the greater Baghdad area — an increase of about 17,500 troops that will double the American force involved in security operations there.
Bush deserves credit for not simply splitting the difference, but we really need to get Casey out of there as soon as we can.