If a blow is going to be dealt to American hegemony, we should look to Europe first. The U.S. is on a perpetual war footing. The Obama administration has picked fights with Russia, China; it is attempting to reassert control in South America, at the same time it is presiding over extensive military campaigns in the Greater Middle East (not to mention all its secret ops and special forces in Africa). The U.S. needs its enabling European sidekick in order to cope with its war mania.
But this war mania is what is splintering the electoral base of Europe's ruling parties. War mania creates refugees who attempt to immigrate to Europe. Merkel is losing consistently in Germany, and Le Pen is on her way this spring in France. Spring is also the time that Conservative Party leader Theresa May announced that Britain will trigger Article 50 and begin negotiating with the EU on an exit, which by statute must be concluded in two years. Spain might be headed for its third election in a year. Italy's Renzi is on the ropes.
At the very least, next year, when Europe is coming apart, it is going to be very hard for the U.S. to bluster and bully as it has for so long. Change is on the way, and it is coming from Europe.