The domestic foreign-policy consensus in the US that has undergirded a half-century of American activism on the international stage is coming under serious strain. Despite the appearance of unified public support for prolonged conflict in a new, twilight fight in Iraq and Syria, there are growing fissures in the foreign policy consensus that cannot be papered over. The alignment that is under threat has shaped American foreign policy for decades. We are entering a challenging period in which the most pressing debates on what to do in the world are playing out inside the two established political parties rather than between them.

The conduct and experience of the Vietnam war fundamentally divided the Democratic party, resulting in a faction that continues to be suspicious of the exertion of American power – particularly military power. This fissure has complicated policy making, but a solid Republican flank helped sustain the postwar consensus about the importance of purpose and engagement. Now a similar split between establishment and insurgent wings within the Republican party is growing with enormous consequences for America’s identity abroad.

The decades-long US support for robust defence spending, for strong security ties with international partners, and for sustaining an open trade system has rested on an unusually enduring alignment between internationalist Democrats and the vast majority of the Republican party. Global posterity and a strong American purpose abroad owe an enormous debt to this uneasy coalition. The isolationist strains of Republican thinking from earlier periods in the 20th century had been virtually nonexistent in party policy making or in Washington’s corridors of power.