The president's role in foreign policy increased largely because?

The role of the President in shaping foreign policy is always at it's highest during a time of War. That being said; terrorism has been cause for a global awareness and allegiance to ward off violence from extremists. The multi-Nationalism now seen in Business is also cause for the President to be concerned about U.S. assets tied to many foreign Nations. It has been a strong policy of all U.S. Presidents to assert the need for every Nation to sponsor and enact a Democratic Society that allows every citizen to have, at the least, basic human rights and a vote for the leadership. World War II and the advent of Nuclear Weapons has formed a fragile and imperative alliance of Nations to keep proliferation and spread of such Weapons from the hands of extremists and Nations which would be considered unstable. In the hands of stable Nations Nuclear Weapons are a huge problem of Global concern. The Economy is based on a Global Market now also; what happens in Tokyo, Singapore, Prague, Moscow, Bangkok, Beijing or London matter as much as what happens in any U.S. City or Market at this point in time. I can be more specific depending on the event or annal in time you wish to explore further. I hope this has helped.