The Trump administration’s motivations for going to war in Iran remain in question. But amid all the debates over the state of the Iranian nuclear program and the condition of the regime, one crucial factor has been overlooked: the United States launched a war more than 6,000 miles from its borders because it could.

States—even those with large and capable militaries—are supposed to struggle to project military power far away. Guns and aircraft have limited ranges, and it is difficult to resupply a distant front. Yet in the past year alone, the U.S. military has struck targets in Venezuela, Nigeria, Somalia, Iran, Iraq, Syria, and Yemen. Since World War II, the United States has fought wars or conducted major combat operations in countries including (but not limited to) Afghanistan, Bosnia and Kosovo, Cambodia, the Dominican Republic, Grenada, Iran, Iraq, Korea, Lebanon, Libya, Panama, and Vietnam. That the United States can bring overwhelming firepower to bear anywhere in the world is something that Americans and the rest of the world have come to take for granted.

The U.S. military has overcome the barrier of distance in large part because countries across the world allow it to use their territory to fight wars. Uzbekistan, for example, let the United States stage the special operations units that invaded Afghanistan in 2001 and Pakistan permitted the logistics and intelligence operations that facilitated and sustained large-scale combat throughout the war. Washington launched its 2003 invasion of Iraq from Kuwaiti territory, and special operations units entered western Iraq via Jordan. When the U.S. Air Force carried out Operation El Dorado Canyon against the Libyan dictator Muammar al-Qaddafi’s regime in 1986, U.S. attack aircraft flew from bases in British territory. Operation Epic Fury is no exception to this long-standing tradition. Host states have allowed the U.S. military to transport its vast combat power through their borders on its way from distant home bases to the Middle East. They have allowed U.S. Navy replenishment ships to load up in their ports to resupply aircraft carriers. And they have allowed the U.S. Air Force to fly sorties from inside their borders against targets in Iran.

When states consider starting a war, one of the factors they consider is whether they can achieve their military objectives at an acceptable cost. Untrammeled wartime access makes it possible for the United States to wage war far away and reduces the costs Washington incurs in the process. The ability to wage war relatively easily around the world can be a good thing. Knowing the U.S. military could get within striking distance could make states otherwise tempted to attack their neighbors reconsider. But increasing the feasibility and reducing the costs of war can also encourage the United States to default to military action, even when it is unclear whether doing so is the best way to advance the country’s political objectives. In short, permissive wartime access increases the odds that the United States wages unwise wars.

After the war with Iran, states may begin to rethink that permissiveness. The United States was unable to shield its Gulf partners fully from Iranian missiles and drones, and other partners may fear becoming similarly vulnerable. If Washington can no longer count on wartime access, it may lose its global reach—but an extra barrier to military action could help keep the United States out of ill-advised wars.

In theory, states should struggle to project military power far away for two main reasons. First, most weapons systems have highly restrictive range limitations. Guns cannot hit their targets across oceans. Aircraft cannot travel very far without refueling, and pilots cannot fly very far before they will need a bathroom and a rest. To use their short-range systems, states must first erase the distance between them and the adversary.

Second, combat operations require support; in military terminology, tooth requires tail. Large-scale combat operations expend huge amounts of fuel and ammunition. Tanks and aircraft need maintenance and spare parts. Soldiers need food and water, medical assistance, and evacuation. Sustaining combat operations requires high volumes of cargo, large numbers of replenishment vehicles to carry the cargo, and crews to operate them. The greater the distance from the depots........

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