This post has been updated.
The United States is bound by a number of treaties that could, in theory, force it to get involved in a war if an ally is attacked. Consider, for example, the situation in Ukraine, a non-member of the North Atlantic Treaty Organization. If a NATO ally were to find itself under similar threat from Russia, the U.S. may find itself duty bound to war.
Or alternatively, cast your mind to the South China Sea and its territorial disputes. If China were to engage militarily with the Philippines at some point in the near future, the U.S. may well be expected to step in to protect its ally: Since 1951, the U.S. and the Philippines have had a bilateral agreement for mutual defense.
It goes without saying that war with either Russia or China would be a very big deal – especially if that war is on behalf of a third party. This becomes more startling when you realize that, thanks to various treaties and deals set up since 1945, the U.S. government might be legally obligated to defend countries containing 25 percent of the world's population.
That figure comes from "The Myth of Entangling Alliances," an article by Michael Beckley, an assistant professor in the Department of Political Science at Tufts University, published this month in the International Security journal.
In his calculations, Beckley used two alliance data sets, the the ATOP dataset and Douglas Gibler’s alliance dataset, to find U.S. defense pacts. To be fair, this results in a broad definition of what constitutes a defense pact: While clearly defense-orientated pacts such as NATO or bilateral agreements are included, the Organization of American States (OAS) also features, even though the OAS is rarely considered a defense pact (the more narrowly focused Inter-American Treaty of Reciprocal Assistance deals with defense and has fewer members).
Beckley also includes two countries where no formal defense agreement has been signed (Israel and Taiwan), arguing that the Taiwan Relations Act and American pledges to support Israel act as de facto pledges of support.
By this count, 69 countries have some form of defense pact with the United States, and as Beckley notes, they make up around 75 percent of the world's economic output. By WorldViews' own count, the combined population of these countries and the United States itself is in excess of 2 billion.
That's a remarkably large amount of the world for the United States to be obligated to protect, especially considering that the country largely kept clear of alliances for the first 165 years of its existence (it did sign one, with France, during the revolutionary war). In fact, America's founding fathers had promised to avoid alliances like this altogether.
“Peace, commerce, and honest friendship with all nations," is how Thomas Jefferson put it in his inaugural pledge. "Entangling alliances with none."
Things really changed after World War II, and even back in 1970, David Fromkin was writing in Foreign Affairs questioning the problems that could be caused by this new web of alliances. Now, in the light of modern situations like those of Ukraine and the South China Sea, some studies have outright suggested these alliances could have disastrous consequences: One article from last year even suggested that America's modern-day alliances resembled the disastrous web of allegiances in Europe before World War I.
Beckley, however, examines the historical evidence and finds that things aren't so cut and dried. He finds only a handful of cases where the United States became entangled in conflicts due to alliances and argues that the conflict that involved the most substantial military action (the Vietnam War) came about due to a result of a variety of factors, not just alliances. Beckley also notes that there are several examples of times in history when the United States has ignored the commitments placed on it by alliances when it didn't suit them: Rebuffing desperate French pleas for overt help during the battle of Dien Bien Phu in 1954, for example.
Perhaps more surprisingly, research also suggests that alliances may keep the U.S. out of war. "In most conflicts, only a few allies were directly threatened and demanded U.S. intervention," Beckley explains in an e-mail. "Most allies, by contrast, urged restraint because they worried their security would suffer if the U.S. drained its strength in a peripheral region or escalated a faraway conflict into a global war."
In the end, Beckley concludes that "U.S. security policy lies firmly in the hands of U.S. leaders and is shaped primarily by those leaders’ perceptions of the nation’s core interests." He is keen to note, however, that that doesn't mean we won't see war against Russia or China at some point in the near future. It's just that it will be political leaders who will decide when to ignore a military alliance – and when to go to war.
You can see a full list of U.S. military pacts below:
Correction: This post has been updated to show that Beckley used two datasets, rather than his own research, to find the the majority of defense pacts he studied. The map attached to this graphic has also been updated to show that New Zealand was no longer part of ANZUS after 1986.