The United States is the only truly global power in all history. The British Empire even at the height of its strength in 1900 could not afford to wield much of its military power so far from its shores as does the US today almost on a whim. Can you imagine the Royal Navy sending its entire fleet of some 50 battleships on a world cruise for publicity purposes, as America did with the famed Great White Fleet of 16 brand new capital ships in 1909? The island nation would likely have been descended on by every jealous European Power with a navy, in the absence of her all-powerful but continental focused Royal Navy.
Much is made today of the rising threat from China, despite the fact that the mostly land-based power is totally obsessed with retaking Taiwan. That tiny country which poses no more military threat against the mainland than does a flea, is more of a political challenge to the communist ideal, that there may actually be a way of life better than theirs, just off their shores. Even with the tiny island democracy out of the picture, can anyone imagine the Chinese taking its eyes completely off its Northern or Western land frontiers for any period of time. Like Britain in her heyday she is mainly concerned with nearby neighbors than she is far away places.
America on the other hand, has easily launched the bulk of her military forces into far-off lands, the distances of major invasion threats to North America giving her this advantage even in the Age of Airpower. Her first such venture was in the Great War, beginning in 1917 when the first elements of the American Expeditionary Force, some 2 million strong, helped defeat the Germans. During the next war in Europe, she repeated this feat, while at the same time launching a simultaneous war against the Japanese Empire in Asia in a Two Ocean War.
This achievement of sending massive military forces far from her shores continued into the Cold War with Korea, and later into Vietnam. The latter Conflict is usually considered our only "lost war", despite the amazing fact that we sustained 1/2 million warriors for several years straight while engaged in constant warfare. Supposedly a fading world power after the Southeast Asian debacle, what did we do in the 1970's but open a third front in the Middle East, in response to the Russian invasion of Afghanistan. That supposed superpower proved its real weakness by fielding less than 200,000 soldiers at any one time to a country which it shared a border with. Meanwhile, President Carter committed over 200,000 soldiers and Marines to defend the Arab states as part of a Rapid Deployment Force, while we still had sizable commitments in Europe and Asia.
Thanks in part to the breakup of the Soviet Union, we could afford to open this Third Front, and launch huge military forces when Saddam Hussein became a threat to the oil fields in 1990. The US committed an astounding 600,000 troops, plus 6 aircraft carriers, 2 battleships, and 2000 warplanes in the largest deployment since World War 2. In the Second Gulf War of 2003, the US utilized half as many forces, thanks in part to modern technology from smart bombs, and the ongoing war there has seen roughly half again as many troops deployed in theater in constant combat.
How long America can keep up its Three Ocean Wars is debatable, especially now that a sizable number of its population is increasingly concerned with continued social reform, including the likely budget breaking idea of Universal Healthcare. With the demise of United States in a hopefully distant future, we may likely see the end of this current global epic, as traditional powers in Europe and Asia return to their own petty, local squabbles.