I would attempt to restore the proper role of the United States Congress as the crucible of our democracy, in which our plans and policies are debated, tested and improved, in order to avoid a repeat of the current situation - where our post-war plans for Iraq were not subjected to scrutiny, and popular support for our Iraq policy is not as robust as we'd like. This would require both of America's major political parties to put the national interest ahead of partisan politics - or, perhaps, would require a renewed sense of civic duty from the American people.
I would make a sharp distinction in the armed forces between warfighting and peacekeeping troops, and develop our capacities for peacekeeping independently of our combat roles. This would probably result in creating a separate peacekeeping entity that had more in common with the Peace Corps than the Army; it would handle the tasks required to create a democracy, vs. those required to destroy a tyrannical regime. It would be centered on the current Civil Affairs and Engineering corps, and would ultimately correct the morale-draining problems we currently have with combat troops assigned to ill-fitting roles.
To the extent possible, I would restore America's influence and support for multinational organizations, including NATO, the EU, and the United Nations, and seek to expand the number of countries involved in the reconstruction of Iraq. By 'involved in the reconstruction' I mean not only materially involved (i.e., sending troops), but also convinced that promoting democracy and stability in Iraq is essential to the well-being of all nations. I would open a sincere dialogue with our allies about the conflict between America's desire to respect international sovereignty vs. our right to protect ourselves from terrorist attacks, and how we may be able to accommodate, or at the very least listen, to the concerns of other nations.