In a speech to the Alexander Hamilton Society last summer, vice presidential candidate Paul Ryan mentioned Aaron Friedberg’s book The Weary Titan. Friedberg’s central argument is that at the turn of the 20th century, Great Britain made a conscious decision to decline—largely because it came to value its domestic welfare state more than its empire.

Congressman Ryan explicitly connected Friedberg’s theme to America’s choice today: “Our fiscal policy and our foreign policy are on a collision course; and if we fail to put our budget on a sustainable path, then we are choosing decline as a world power.”

Some analysts have highlighted how Ryan’s 2012 defense budget proposal kept President Obama’s defense budget cuts intact. And in 2013, Budget Chairman Ryan is proposing only modest increases in military spending relative to the president.

Most important in the battle over budgets, priorities, and government is not the differences between Ryan and Obama’s defense budgets but rather Mr. Ryan’s laser-like focus on the urgent need for entitlement reform. Without an honest effort at reining in costs and putting the Big 3 entitlements on more solvent footings, the debate about the shrinking defense budget will simply be overtaken by the math of America’s debt.

This is already happening. The defense budget is set to be eclipsed by interest payments on our national debt at some point in the next five years. Meanwhile, some are projecting that Medicare Part A could run out of money within the same timeframe, absent change.

Regardless of the threats facing our country, the requirements of our military, or the strategy of any administration, the defense budget will be increasingly beholden to what politicians do or not relative to automatic and growing mandatory spending on Social Security, Medicare, and Medicaid—roughly two-thirds of our federal budget.

Mr. Ryan seems to be betting that the true culture war that will define America’s future is not about social issues but spending. America cannot continue to fund entitlements without reform while playing a leading role in global affairs.

By attempting to put entitlement spending on a sustainable course, Mr. Ryan is attempting to actively maintain America’s role in the world. Without reforms on the scale of what he is proposing, America will eventually be unable—and perhaps unwilling—to meet its global commitments. It is clear that Mr. Ryan’s national security credentials are indeed genuine.

Mackenzie Eaglen is a resident fellow in the Marilyn Ware Center for Security Studies.