President Obama travels to Europe this week, a trip that will include visits to Ireland, France, and Poland. His visit to the United Kingdom, however, holds special importance.

While in London, President Obama is sure to talk with British Prime Minister David Cameron about the president’s latest proposal for the Middle East and North Africa, aspects of which the British may well support. The global economy will surely be on the agenda. The most important discussions, however, should focus on the two wars that involve both the U.S. and British militaries (Afghanistan and Libya).

There is nothing novel about a high-level meeting between U.S. and British leaders. They happen all the time. Beyond the obvious historical and cultural connections that bind the two countries together, the special relationship has been cemented by the numerous occasions over many decades in which British and American leaders have cooperated to address common security challenges. The most important of these involve U.S. and British troops fighting side by side. As Jason Davidson notes in his just-published book, America’s Allies and War (that we featured at Cato earlier this month), British PMs place a great deal of importance on maintaining close ties with the United States. That doesn’t mean that they always send troops to support U.S. military missions: Harold Wilson refused to help out in Vietnam; and John Major turned aside George H.W. Bush’s request for help in Somalia. The British, however, have stepped up more often than other U.S. allies.

A limiting factor is the number of British troops, ships and planes that are available for such missions. These numbers have been dropping for some time, and they will drop still further in coming years. Cameron is pushing significant cuts in British military spending—actual cuts, not the slowing in the rate of growth that Obama and Defense Secretary Gates have presided over so far. Cameron, a Tory, is portrayed as determined to rein in British spending, but military spending constitutes just five percent of the UK’s budget, and other spending—especially foreign aid—is slated to grow. In comparison, about 20 percent of federal spending in the United States goes to the military.

Total U.S. military spending might actually decline next year, especially if the U.S. troop presence in Afghanistan is drawn down quickly. But if that does happen, and the total DoD budget settles around $671 billion (the base budget, plus the cost of the wars), Americans will still spend more than twice as much as the British as a share of GDP, and more than two and a half times as much on a per capita basis. The British still spend slightly more than the French, both in real terms, and on a per capita basis, but France may eclipse the UK within the next few years.

So, the United States today spends far more on its military than does the United Kingdom, and the gap will likely grow in the coming decade. Where will this leave the U.S.-UK relationship? It seems unlikely that the special relationship will be so special when the Brits must politely decline future U.S. requests for military assistance in dealing with common security challenges. The open question is whether U.S. leaders will be as willing to assist the British as President Obama was when Cameron (and French President Nicolas Sarkozy) asked for, and secured, U.S. military intervention in Libya.

As British defense spending shrinks, will the goodwill that has prevailed between the two countries persist, and will Americans still look upon the Brits as the "good" ally, the one that sticks by us through thick and thin? And if the American public grows disenchanted with British contributions to U.S.-led military missions, will the British public still hold generally positive opinions of the United States? The most recent Pew Global Attitudes survey finds that 65 percent of British respondents have a favorable view of the United States, placing them just between Japanese (66 percent) and Germans (63 percent). Notably, 73 percent of French respondents had a favorable view of the United States; South Koreans were among the highest at 79 percent.

I predict that the special relationship may change if other countries grow more willing to step forward, both in defense of their own interests, and in their respective regions of the world. If our allies expanded their military capabilities—even very modestly—they would be better able to assist us, if we were ever genuinely in need. The British, however, are moving in the opposite direction, cutting their military spending, and tacitly becoming more dependent upon U.S. power to safeguard their interests, and their security.