" " U.S. President Barack Obama speaks during a press conference at the OECD Conference Centre before leaving the Climate Change Conference COP21 on Dec. 1, 2015, in Paris. Pascal Le Segretain/Getty Images

­In 1823, Pre­sident James Monroe, in the face of new republics emerging in Central and South America from former Spanish colonies, called for Europe to remain out of the Americas and not to intervene in the nascent countries. This decree, called the Monroe Doctrine, essentially divided the world into two hemispheres, with the United States at the helm of the West.

In 1963, President John F. Kennedy signed an executive order that transferred command of the Alabama National Guard out of the hands of Gov. George Wallace and into his. Under Kennedy's authority, the National Guard was ordered to protect two black 20-year-old students, James Hood and Vivian Malone, as they entered the campus of the University of Alabama to enroll and begin desegregation of the school.



In 1964, most elderly and poor Americans didn't have health insurance. That year, President Lyndon Johnson issued his Great Society social reform package. The Great Society, among other things, created the federal health care programs Medicare and Medicaid. By 2003, one-quarter of all Americans had access to health care through the programs [source: De Lew].

In the spring of 2008, riots broke out in the Philippines, Haiti and Egypt as ­food prices rose dramatically, limiting access to basic staples by the poor in those countries. In response, President George W. Bush ordered a member of his cabinet, Agriculture Secretary Ed Schafer, to release $200 million in emergency funds for additional food aid. The funds helped to quell the unrest in places like Manila, Port Au Prince and Cairo and feed hungry citizens of those nations.

­The actions of each of these men and all othe­r presidents helped shape the world as we know it today. Often, these world-changing events resulted from a simple signature or a public address. But none of these changes could have been made were it not for the one thing each of these people had in common, that each was president.

­How did this office become so powerful? How has it changed over time? In this article, we'll get to the nuts and bolts of how the U.S. presidency works and its evolution from the head of a nation to arguably the leader of the free world.