Fiscal irresponsibility is politically attractive, but it is equivalent to believing in something for nothing. Basing the policy of the world’s dominant economy on the hope that the normal rules of fiscal prudence do not apply is an exceedingly dangerous idea.

Large and sustained deficits in the United States threaten not only U.S. prosperity but the world’s economic health as well. Massive public borrowing in the United States is already absorbing other nations’ savings to finance the world’s richest country. And it may soon raise interest rates around the world and slow global growth. U.S. profligacy could even invite an international financial crisis that would bring enormous human costs everywhere.

Small countries cannot afford to behave irresponsibly for very long; their currencies lose value and their governments cannot borrow money. But investors give the United States more leeway. Its debt—the famed U.S. Treasury bonds—is still regarded as a very safe place to park money. The persistent appeal of U.S. bonds is leading politicians in the United States to believe that the ordinary rules of global finance don’t apply to them. When they realize that rules are rules, it may be too late; the world could be caught in a financial crisis that has escalated beyond control.

Sermons on fiscal rectitude often fall on deaf ears in the United States. Everyone likes a free lunch if they can get it. Raising taxes and cutting spending are always painful, and political leaders have to be convinced that the pain is worth it. But a glance at the recent past should wake the slumbering body politic.

In the early 1980s, the Reagan administration cut income tax rates and increased defense outlays without restraining other spending. Supporters of those tax cuts predicted they...