As the U.S. economic expansion ages and clouds gather overseas, policy makers worry about recession. Their concern isn’t that a downturn is imminent but whether they will have firepower to fight back when one does arrive.

Money has been Washington’s primary weapon in the decades since British economist John Maynard Keynes proposed aggressive government spending to battle the Great Depression. The U.S. generally injects cash into the economy through interest-rate cuts, tax cuts or ramped-up federal spending.

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