Britain is already much farther down that road. In 1965, Britons spent a little less than 7 percent of GDP on defense and nearly 11 percent on entitlements, according to figures compiled by the Heritage Foundation. Today, they spend about 3 percent on defense and more than 23 percent on entitlements. British leaders are contemplating even more defense cuts in the future; by then, they might not be able to afford even a few cruise missiles.
It's not clear that Ryan's proposed budget is precisely the right way to solve the entitlement problem. The same is true for Barack Obama's deficit commission (whose advice the president ignored). But so far, Ryan and his conservative colleagues, plus the deficit commission, are pretty much the only players in the game. Democrats who attack Republicans for advocating entitlement reform without having any solutions of their own have nothing to add.
None of this is to say that a strong defense must come at the expense of social spending. "It's a false choice between being a compassionate nation and being a military superpower," says Heritage Foundation research fellow Mackenzie Eaglen, who tracks defense issues. "If done responsibly, a nation can do both."
It's also not to say that military spending should never be cut. There remains a lot of waste and redundancy at the Pentagon. At his news conference, Ryan endorsed the cuts -- nearly $80 billion -- that Defense Secretary Robert Gates has proposed.
Defense spending grew a lot in the past decade (though still not as fast as entitlements), but that was mostly because of ongoing operations in Iraq and Afghanistan. That's different from the Reagan defense buildup of the 1980s, which was an investment in new systems for the future. In the next few years, facing an entitlement-fueled runaway deficit, there will be many calls for the United States to cut back on military spending. A look at once-mighty Britain shows where that could lead.