BENJAMIN M. FRIEDMAN: William Joseph Maier Professor of Political Economy, Harvard University, and author, Religion and the Rise of Capitalism (forthcoming).

With the launch of a coordinated union-wide spending program, financed by the first-ever issuance of union-obligated bonds, has the European Union finally taken the decisive step toward becoming a genuinely unified fiscal entity? Americans should certainly hope so. European unification has been a centerpiece of American foreign policy thinking since the days of Stimson and Kennan and Acheson. The motivation then was unity to safeguard the continent against the threat of Stalinist aggression. Today it is to avoid the potential conflict due to resurgent nationalisms, a threat exacerbated by the European Union's inclusion in recent years of several increasingly authoritarian states, each led by its own mini-Trump. But the objective remains sound, and European patriots should aspire to unification no less than Americans wish it for them.

While the primary rationale for European unification is political, the cutting edge of progress on this front has always been economic. The Coal and Steel Community of the early post-war years gave way to the Common Market, then the Economic and Monetary Union, and in turn the European Union itself. A central bank followed soon after, giving most European countries a common currency and monetary policy, and most recently a banking union with a single supervisory system and mechanism for resolution in case of failure (although not yet with serious money behind it). But so far--until now--no real unity in fiscal matters.

Many of us have long thought that this final step would come only in response to some crisis. I expected the 2008-2009 financial crisis, with the threat of widespread collapse in the banking system, to be the trigger; I was wrong. Now the severe...