The US is so powerful for reasons other than its size, its military might, and its global system of alliances and bases — although those are certainly important. There is also America's tremendous advantage in scientific research, which both furthers and is an expression of its technological and economic lead on much of the rest of the world; it's also an indicator of innovation more broadly. An imperfect but revealing shorthand for that is the US's tremendous lead in Nobel prizes from its 1901 inception through 2013, when I made this map (the US has not lost its Nobel lead since then). The US has won 371 Nobels, mostly in the sciences; the US thus accounts for 4 percent of the world population but 34 percent of its Nobel laureates. This is the result of many factors: wealth, a culture and economy that encourage innovation, education, vast state- and private-funded research programs, and a political culture that has long attracted highly educated migrants. All of those factors contribute to American wealth and thus power in more ways than just Nobel prizes, but the sheer number of US laureates is a sign of the American advantage there.
Because of a war that left North America vulnerable to British conquest — and thus ready for the US's expansion.
Another way to show America's status as the sole global superpower is its military budget: larger than the next 12 largest military budgets on Earth, combined. That's partly a legacy of the Cold War, but it's also a reflection of the role the US has taken on as the guarantor of global security and the international order. For example, since 1979, the US has made it official military policy to protect oil shipments out of the Persian Gulf — something from which the whole world benefits. At the same time, other powers are rapidly growing their militaries. China and Russia in particular are rapidly modernizing and expanding their armed forces, implicitly challenging global American dominance and the US-led order.