Navies are expensive, and sending warships to Somalia is a hugely inefficient way to fight pirates, considering that the number of successful attacks off the Somali coast this year — 35 by mid-November — is only seven below the total for all of 2008, before NATO and the EU had anti-pirate missions in the region.

So why are they there? The short answer is that Western governments don’t know what else to do. But the U.S. and Europe also have different self-interested reasons to cruise the Indian Ocean.

For most of the European governments involved, the obvious idea is to protect their domestic shipping business. “For these big shipping nations, it’s not a big deal,” a Horn of Africa expert named E.J. Hogendoorn, at the International Crisis Group in Nairobi, told me in September. “They’ve got big shiny navies; what the hell are they gonna do with ‘em anyway? Might as well park ‘em off the coast of Somalia. It’s as good a training as any.”

And defending the sea lanes is more or less what everyone’s claiming to do. But America’s interest is more complicated. In spite of the dramatic rescue of the U.S.-flagged Maersk Alabama last April, the ocean isn’t terribly crowded with American merchant vessels. (Even American shipping lines sail under foreign flags, to save money.) The real draw to this part of the world, for Washington, is counterterrorism.

The Navy won’t say so. “Piracy’s an international problem requiring an international solution,” said Lt. Matt Allen, a spokesman for the Fifth Fleet in Bahrain, where the separate U.S.-led counterpiracy coalition (Combined Task Force 151) is based. “Even though a small portion of the [merchant] vessels are U.S.-flagged, a majority are allies and friends. Every nation has a vested interest in insuring the safe passage of the sea lanes.”

But Washington’s military buildup on the Horn of Africa started with the war in Afghanistan, which led to a (still-growing) naval base in Djibouti, just north of Somalia. Officially, the base in Djibouti has nothing to do with pirates. It’s lodged inside what the Pentagon calls an “Arc of Instability” stretching from Kenya to Yemen. Al-qaeda and some like-minded groups have civil wars running in Yemen, Somalia and Sudan — and Somalia, of course, could become “the next Afghanistan” if Islamists like al-Shabaab take over. Washington wants to watch these developments.

In September, a small American team killed a long-wanted terrorist named Saleh Ali Saleh Nabhan south of Mogadishu, which suggests that the Pentagon has good intelligence in the region. No one in Djibouti, Bahrain or the AFRICOM headquarters in Germany will admit to organizing the raid, but Special Forces from an American warship reportedly helped, which might indicate a measure of support from the Navy’s counterpiracy base in Bahrain (since the Djibouti base doesn’t send out warships, officially).

The Navy’s new jet-sized surveillance drones based in the Seychelles — officially to watch pirates — also have more than enough range to glide over Somalia and the rest of the Pentagon’s “Arc of Instability.”

But there’s a third interest at stake in the fish-rich Indian Ocean. Some European fishing boats wander down from the Mediterranean (and away from hated EU regulations) to trawl the lawless coast of Somalia for tuna, lobster, shrimp and shark.

“It is particularly ironic that many of the nations that are presently contributing warships to the anti-piracy flotillas patrolling, or set to patrol, the waters off the Horn of Africa, are themselves directly linked to the foreign fishing vessels that are busily plundering Somalia’s offshore resources,” Clive Schofield, an Australian research fellow at the University of Wollongong and author of a paper called Plundered Waters: Somalia’s Maritime Resource Insecurity, has written.

Namely: France and Spain. Hogendoorn, at the International Crisis Group, singled out Spain. “I’ve spoken to diplomats in Europe who’ve made it quite clear that Spain has been very active in the piracy issue because of its own national interests,” he said, “which can only be interpreted to mean that they have fishing vessels making lots of money off of fishing in Somali waters.”

Decimation of Somali fishing is a major complaint of the pirates themselves. The notion of a Somali fisherman hijacking a cargo ships because of collapsing fish populations is over-simple — piracy is organized crime — but the complaints about systematic decimation of African fish is real. Many Somalis, in fact, think the warships they see from their beaches have arrived to make the seas safe for foreign boats.

Aboard a NATO frigate in September, a British officer, Lt. Cmdr. Graham Bennett, noticed the lack of fishermen out on a calm, sunny day. It was the start of fishing season in the Gulf of Aden — the monsoons had just ended — but Somali fishermen seemed to be staying home.

“We need to get the word out that we’re not here to arrest everyone,” he told me. “We got on Somali TV the other day [and] said we really do want to protect the fishermen themselves from piracy. But some of the Somali people thought we were just here to protect the European fishing trawlers. That surprised us a little.”

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