He’d invade Pakistan. From the Telegraph:

“There are terrorists holed up in those mountains who murdered 3,000 Americans,” Mr Obama will say at the Woodrow Wilson International Centre for Scholars.
“They are plotting to strike again. It was a terrible mistake to fail to act when we had a chance to take out an al-Qa’eda leadership meeting in 2005. If we have actionable intelligence about high-value terrorist targets and President Musharraf won’t act, we will.”