After WWII Truman entered the U.S. into a series of treaties and conventions which limited American action outside of international agreements and shared policies and thereby allowed us to escape the traditional fate of the world's leading power: fear and loathing of unilateral, arbitrary action.
The U.S. had more power and influence in the world during the Cold War years precisely because it had agreed to act in concert with its allies; it constrained its power, so then everyone felt comfortable with its power.
The Nonconservatives rejected the Truman Doctrine, basically saying the U.S. should be free to act as it saw fit without the entangling fetters of allied interests. President George W. Bush broke with the policies of every president since Truman and moved America outside of these long standing treaties and conventions, creating ad hoc alliances based on unilateral American action.
It was a disaster. The unilateral actions in Iraq left America with sole responsibility for the cost and casualties there. The withdrawal from the Kyoto treaty left America out of the discussion entirely. America's foreign policy initiatives were met with the traditional fear and loathing the world's leading powers in the past centuries had experienced.
Now, President Obama is restoring the Truman Doctrine to the center of American foreign policy; by acting in concert with allies and committing to comply within internationally accepted standards, America will regain its proper role of influence in the world without the fear and loathing the Bush Doctrine incurred.
I don't doubt that there are doctorates in international studies being written about this history, but, for this amateur, in a nutshell that's why I think Obama's declaration this week was important and a good thing for our country.