If we want to reduce the threat of international terrorism--especially that which is inspired by the Muslim perception that they and their faith are under assault, we should force Israel to end their brutal and illegal 41 year military occupation of Palestine. By doing so, we would also vastly improve the image of the US in the eyes of the rest of the world because that would suggest that we may finally have decided to be committed to the rule of law and to justice for the oppressed.
The Palestinian issue is the primary cause of anti-American terrorism. The US must accelerate the formation of a free and independent Palestine w/o Israeli settlements, w/o Israeli control of the water, and w/o Israeli blocking access to cropland. The Illegal settlements MUST be turned over to Palestine intact as well as the connecting roads as reparation. We need to build their infrastructure: roads, electricity, sanitation, port facilities, schools, hospitals, irrigations systems, etc.
The U.S. need to acknowledge that the 9/11 attacks were done by a Saudi-based group and had nothing to do with Iraq, distance itself from Israel and Palestine whose leaders can't or won't persuade everyone of their citizens to get along with each other, and step up looking for bin Ladin. We also need to bring fair elections back to this country on a national level; local elections seem to be run fairly.
On Day One, the president should start a new and open foreign policy, including everything from working for peace in Israel-Palestine to lifting the embargo on Cuba, so that the entire world knows that America is back and willing to work with the rest of the world in tackling international problems that we can't possibly tackle on our own.