Here's What You Need to Remember: Whether the Iranian military could defeat U.S. or allied task forces is an open question. It is beyond question that Tehran can impose heavy opportunity costs on Washington. It’s already doing so. After all, every gray hull facing down the IRGCN or regular Iranian Navy is a gray hull not facing down Chinese or Russian fleets or pursuing other worthwhile endeavors such as training, scraping rust, or relaxing in home port.

That chronic pain gnawing at officialdom’s guts is bipartisan. Presidential administrations, Republican and Democratic alike, keep trying to draw down the U.S. military presence in the Middle East, the Persian Gulf region in particular, to attend to more pressing priorities. Back in 2012 the Obama administration vowed to “pivot” or “rebalance,” from the Middle East to the Pacific theater to counterbalance China. President Donald Trump and his lieutenants proclaim that an age of great-power competition is upon us. Like their Democratic forerunners, they have signaled their desire to reapportion finite U.S. diplomatic and military resources elsewhere around the Eurasian perimeter—say, to the South China Sea or Baltic Sea.

This is sound strategy. Strategy is about setting and enforcing priorities. Lesser priorities must yield to greater lest a competitor exhaust itself trying to do everything, everywhere. Not even superpowers are exempt from this iron law of world politics.

But if U.S. presidents prefer to compete against China and Russia, the Gulf region stubbornly refuses to let America and its allies leave. Iran is the foremost mischief-maker. Whether out of strategic calculation, ideological fervor, or plain orneriness, the clerics who govern the Islamic Republic appear bound and determined not to let the Great Satan vacate their backyard. Running feuds over nuclear-weapons development and economic sanctions, freedom of maritime movement through the Strait of Hormuz and its environs, and drone shootdowns rank among the headline-grabbing issues miring the United States in the Middle East. Seldom, of late, does a day pass without some bitter exchange between Tehran and the West.

Much of the action has transpired at sea or in the skies overhead. This past Thursday, USS Boxer, a light aircraft carrier designed for amphibious operations, shot down an Iranian drone that approached to within one thousand yards of the vessel—presumably along a menacing intercept course. The downing took place scant weeks after Iranian anti-aircraft artillerymen brought down an American drone flying along the Iranian seacoast.

Tehran has repeatedly pledged to close the Strait of Hormuz to surface traffic and has publicly toyed with the idea of charging ships a toll to traverse the narrow waterway. The leadership disclaimed responsibility for a recent spate of attacks on merchantmen in the Gulf of Oman, along the southern approaches to the Strait. This week, however, the Islamic Revolutionary Guard Corps Navy (IRGCN), the Iranian irregular naval force, seized a Panamanian-flagged tanker based in the United Arab Emirates. Spokesmen subsequently accused the crew of MT Riah of smuggling Iranian oil. The seizure came mere days after a British frigate shooed IRGCN vessels away from a British tanker transiting Gulf waters. In turn that encounter constituted Tehran’s reply to British actions in the Mediterranean Sea, where Royal Marines detained an Iranian supertanker allegedly bound for Syria in breach of European Union sanctions.

Tit for tat.

The U.S. Navy keeps potent forces on station in an effort to manage events in Middle Eastern waters and back diplomacy with steel. Task forces centered on Boxer and the nuclear-powered aircraft carrier Abraham Lincoln are currently operating in the region under the aegis of the Bahrain-based U.S. Fifth Fleet, the U.S. Central Command’s naval arm. That’s a sizable fraction of U.S. naval power for a theater Washington longs to demote on its strategic agenda. Bear in mind that on a good day the navy has just four nuclear-powered flattops like Lincoln (of eleven in the inventory) fully ready for action along with three amphibious carriers like Boxer (of eight total). The remainder are working up for combat duty, recovering from extended deployments, or undergoing maintenance or overhauls.

That means two of seven naval-aviation hulls are executing duties in or around the Gulf while five are entrusted with the rest of the globe. Tehran, it seems, has managed to entangle the world’s leading superpower in a theater it would like to be quit of; done so at low cost by employing light naval forces; exacted a high price from the superpower for the privilege of remaining in that unloved theater; and siphoned away resources the superpower needs for strategic competition in more crucial theaters. Small wonder deputy Revolutionary Guard Corps commander Admiral Ali Fadavi crows that when foreign ships “enter the Persian Gulf they say among themselves, ‘we just entered hell.’ And whenever they exit the Persian Gulf, they say, ‘we went out of hell.’”

Hell indeed in strategic terms. Whether the Iranian military could defeat U.S. or allied task forces is an open question. It is beyond question that Tehran can impose heavy opportunity costs on Washington. It’s already doing so. After all, every gray hull facing down the IRGCN or regular Iranian Navy is a gray hull not facing down Chinese or Russian fleets or pursuing other worthwhile endeavors such as training, scraping rust, or relaxing in home port.

The Islamic Republic and Britain may be squabbling at the moment, but Iranian strategy exhibits a strikingly British flair. Britain during the age of sail, that is. In 1808 the Royal Navy landed an expeditionary army on the Iberian Peninsula, Napoleonic France’s western flank. Having swept the French and Spanish navies from the sea at Trafalgar in 1805, Britain’s navy could mount amphibious operations in European rimlands with near-impunity. Beneficiaries of logistical support from the sea, groundpounders commanded by Sir Arthur Wellesley—later elevated to duke of Wellington—battled French forces in concert with Portuguese and Spanish partisans. This hybrid campaign bled France for the next six years. So successful was Wellington’s venture that allied forces ultimately broke into France and helped compel Napoleon to abdicate.

Overseers of the Peninsular War had no particular strategic or political goal in mind when they conceived the campaign. They simply allotted Wellington a humble 50,000-man army and a supporting Royal Navy fleet. They bade the expeditionary force go forth and make trouble for the little emperor in a theater he wished would remain quiet and undemanding in manpower terms. So impactful was the British strategy that Napoleon joked sardonically about his “Spanish Ulcer.” It inflicted less-than-fatal but constant nagging pain, distracted attention and energy from more important affairs, and drained resources that should have gone into the main fighting theater to France’s east. Best of all from the allies’ standpoint, it accomplished all of this at a bargain-basement price.

The peninsular campaign returned gains disproportionate to the investment—the hallmark of effective strategy. It was what naval historian Julian Corbett, channeling Carl von Clausewitz, called a “war by contingent.” Normally policymakers set strategic goals for an enterprise, allocate martial means sufficient to achieve those goals, and measure progress toward them, adjusting the effort when necessary. War by contingent is not a goal-driven strategy; it is a resource-driven strategy. Policymakers supply a force they can spare without placing more crucial theaters in jeopardy—Corbett terms it a “disposal force”—and order that force into the field to sap enemy resources and resolve as its commanders think best. This is not a war-winning approach in itself. But a disposal force, artfully handled, can enfeeble the foe in the main combat theater. A token force becomes a difference-maker in the larger struggle.

Iranian strategy echoes Wellington’s strategy but adds a twist, or rather several of them. First of all, Tehran can wage war by contingent on the cheap. Nearby seas, not some distant shore, constitute the scene of strife. Iranian military folk can project power seaward using light forces such as land-based missiles and aircraft and speedboats toting guns or missiles. Unlike Wellington’s host in 1808-1814, they need not command the sea before staging a war by contingent. They can cause problems from home territory even though the U.S. Navy and its allies rule the waves. Such measures keep hostile navies on edge while driving up the price of oil and gas as Tehran deems fit. Insurance rates for shippers go up when shipping comes under threat. Markets likewise tend to spike. The new costs are passed on to consumers, who may bring pressure on elected leaders to soften their line vis-à-vis the Islamic Republic. Political and strategic advantage accrues.

Second, the mullahs need not fret that pursuing a war by contingent will place more critical theaters at risk. Managing Iran’s marine near abroad is the ruling regime’s top priority. That is the same setting where its war by contingent unfolds. Ultimately Iranian rulers, unlike Bonaparte, have no decisive theater of action. At most they can hope their opponents will tire of ceaseless struggle and strike an accommodation on Iranian terms—or go away altogether. While that narrows Tehran’s options for seeking victory, it also simplifies operations: the disposal force is the main Iranian force. Commanders and their political masters can dispatch as much or as little of that force as they choose on any given day—and thus dial up or down the effects of their war by contingent as circumstances warrant. Such an enterprise uncoupled from a main effort somewhere else doesn’t add up to a war-winning strategy. It is an excellent strategy for harnessing meager resources if mischief-making represents the goal.