During his successful presidential campaign, Donald Trump argued that the United States gets ripped off by its partners. He threatened to withdraw U.S. forces from overseas and to abandon America's global role. In short, the author of "The Art of the Deal" threatened to shred the deal that underpins the welfare of not only his own country but also of its allies and in many ways the rest of the world. As is often the case with him, Trump was short on specifics. But it was enough to terrify many in Tokyo and in other capitals, including Washington.
Since the early Cold War, the core allies of the U.S. have stuck to a win-win bargain with Washington. The U.S. provides a larger proportion of its resources to the common defense in exchange for a leadership position far above what the size of its GDP would warrant. The others willingly surrender part of their sovereignty to Washington.
The Americans permanently lament that their partners aren't sharing the burden; the allies in turn continuously complain about the U.S. But this combination of greater direct economic costs for the U.S. and less sovereignty for its partners is logical. It makes this global coalition, which includes Japan, South Korea, Australia/New Zealand, NATO and a few other de facto allies, more efficient. It explains why practically no country has left the U.S. alliance system. As the painful travails of Napoleon's foes showed, coalitions without hegemons are ineffective. This U.S.-dominated structure has overcome quite a few serious but not mortal failures, the arrival of new parties in power, and the test of time. It has evolved and must do so, but its basic architecture is sound.