I think Powell would need to have an extremely popular VP, most likely someone with the right name recognition, and also (to gain the hard red states) old and white, most likely someone like John McCain. I say that, mostly because during the 2008 election, Powell was considered as a potential VP candidate before Sarah Palin.
When it comes to situations like the War on Terror, he may have focused the War on Terror primarily on destroying Al Qaeda, and capturing or more likely killing Osama Bin Laden.
He and McCain (Both Vietnam veterans) would already know what its like for the American people to fight a long war, and know that the will of the American people can only stay strong for too long, so no "Weapons of Mass Destruction" speech at the UN, or an invasion of Iraq in 2003.
This however would not stop the Arab Spring, or similar conflicts, so the War on Terror, like Judgement Day in the Terminator franchise, is inevitable (if you count T3 as cannon), but may happen either near the ladder half of his second term (If he wins one), or around the start of someone else's administration, and may most likely be fought primarily with predator drones, and bombing strikes.
He also seems to be quite bipartisan, willing to cooperate with the democrats, which would help to gain the votes of people who would usually vote democrat. He also has some Liberal view points, including pro-choice, reasonable gun control, and even supported the repeal of the don't ask, don't tell policy. Although the ladder may be used against him by his rivals, seeing how he was instrumental in its implementation.