Key point: The German government has just recently given the go-ahead for yet another set of three more Dolphins.

Israel’s submarine corps is a tiny force with a big open secret: in all likelihood, it is armed with nuclear weapons. The five Dolphin-class submarines represent an ace in the hole for Israel, the ultimate guarantor of the country’s security, ensuring that if attacked with nukes, the tiny nation can strike back in kind.

Israel’s first nuclear weapons were completed by the early 1970s, and deployed among both free-fall aircraft bombs and Jericho ballistic missiles. The 1991 Persian Gulf War, which saw Iraqi Scuds and Al Hussein ballistic missiles raining down on Israeli cities, led Tel Aviv to conclude that the country needed a true nuclear triad of air-, land- and sea-based nukes to give the country’s nuclear deterrent maximum flexibility—and survivability.

The most survivable arm of the nuclear triad is typically the sea-based one, consisting of nuclear-armed submarines. Submarines can disappear for weeks or even months, taking up a highly classified patrol route while waiting for orders to launch their missiles. This so-called “second-strike capability” is built on the principle of nuclear deterrence and ensures potential enemies will think twice before attacking, knowing Israel’s submarines will be available to carry out revenge attacks.

The first three submarines were authorized before the Gulf War, in 1988, though it is not clear they were built with nuclear weapons in mind. After years of delays construction began in Germany instead of the United States as originally planned, with German combat systems instead of American ones. Most importantly, the project went ahead with German financing; Berlin reportedly felt obliged to finance two of the submarines, and split the third as lax German nonproliferation enforcement had partly enabled Iraq’s nuclear and chemical weapons program.

The first three submarines, Dolphin, Leviathan and Tekuma, were laid down in the early 1990s, but only entered service between 1999 and 2000. The submarines are 187 feet long, displace 1,720 tons submerged and have an operating depth of 1,148 feet. Sensors include the STN Atlas Elektronik CSU-90-1 sonar suite with the DBSQS-21D active and AN 5039A1 passive sonar systems. The Dolphin class also has PRS-3-15 passive ranging sonar and FAS-3-1 passive flank arrays.

Each has ten torpedo tubes in the bow, six standard 533-millimeter standard diameter tubes and four larger 650-millimeter torpedoes. The larger torpedo tubes are more than two feet wide, and reportedly double as ingress/egress chambers for divers. Armament is a mixture of German, American and Israeli weapons, including Seahake heavyweight wire-guided torpedoes and Harpoon antiship missiles. The authoritative Combat Fleets of the World claims the Dolphin subs may have the Triton fiber-optic guided-weapon system. With a range of more than nine miles, Triton allows submarines the ability to attack helicopters, surface ships and coastal targets.

The four large torpedo tubes are the key to Israel’s sea-based deterrent, and without them it’s unlikely the country would have nukes on submarines. The large tubes are used not only for laying mines and sending and receiving divers, but also to launch nuclear cruise missiles. In 2000, the U.S. Navy observed a missile launch from off the coast of Sri Lanka that traveled an estimated 932 miles. Exactly what this missile was is a matter of speculation, but the leading candidate is some advanced form of the Popeye missile.

Popeye was originally an air-launched ground-attack missile. Developed in the late 1980s, Popeye originally used a television camera or infrared seeker to deliver a 750-pound warhead to ranges of up to forty-five miles. The United States Air Force bought 154 Popeye missiles to arm B-52 bombers for conventional attacks, renaming them the AGM-142 Raptor. Israel’s nuclear deterrent is thought to be based on cruise missile version of Popeye, Popeye Turbo, which has a turbofan engine for long-distance flight.

There is also the possibility the nuclear armament is based on the Gabriel antiship missile, and there are also reports that Harpoon missiles were modified to carry nuclear weapons. Nobody appears to know for sure what missile is operational, only that it was observed and that arming them with nuclear weapons is a logical conclusion. The yield of the nuclear warhead on these missiles is unknown, but estimates float around the two-hundred-kiloton mark, which would make them roughly fourteen times more powerful than the bomb dropped on Hiroshima.

Whatever the missile, a 932-mile range gives it the ability—just barely—to strike the Iranian capital of Tehran, as well as the holy city of Qom and the northern city of Tabriz, from a position off the coast of Syria. (Iran’s pursuit of nuclear arms is likely the main and enduring driver of Israel’s second strike capability.) That isn’t an ideal firing position, and it’s been seventeen years since the missile’s first flight, so it’s also reasonable to assume that the weapon’s range has been extended to the point where it can launch against Tehran and even more Iranian cities from a relatively safe location.

Having three submarines in operation generally means at least one is at sea at any particular time, a necessity for a sea-based nuclear deterrent. The Dolphin class reportedly carries up to sixteen torpedoes and missiles; if the submarines’ primary task is nuclear deterrence, half of its weapons space might be allocated for carrying nukes. The result is that at any given time Tehran is likely in the nuclear crosshairs of an Israeli submarine.

The second set of Dolphin submarines, Dolphin II, was ordered in the mid-2000s. These subs are virtually identical to the previous class except for the addition of a thirty-six-foot-long plug in the hull to accommodate an air independent propulsion (AIP) system, allowing the submarine to operate submerged for much longer periods than diesel electric subs without it. According to Der Spiegel, the Dolphin II subs can stay underwater for up to eighteen days. In addition to a stretch configuration and AIP, Dolphin IIs weigh approximately 20 percent more and have dedicated diver-lockout chambers.

The German government has just recently given the go-ahead for yet another set of three more Dolphins. These new submarines should be ready just as the three first-generation boats are aging out, ensuring that Israel has a fleet of six submarines available for the foreseeable future. Israel’s sea-based nuclear deterrent is here to stay.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This article first appeared earlier this year.

Image: Flickr.