Pirates have it rough. In order to board a cargo ship, they need to get within range using a pirate "mothership," typically a hijacked fishing vessel. Once close enough, the mothership launches several small, high-powered skiffs. These skiffs close the distance with the cargo ships, typically traveling through the cargo ship's tumultuous wake to avoid detection. Once alongside the ship, the pirates then have to use grappling hooks or ladders to climb to the deck; not so easy when you've got an AK-47 slung across your back and the cargo ship's crew is desperately trying to douse you with fire hoses.

That's the old Somali model, popular around a decade ago. But current-day Houthi rebels have used technology to change the game. Because they're driven by ideology and not profit, the Houthis don't need to board the ships; they just need to mess them up. They've thus been early adopters of Uncrewed Surface Vehicles (USVs), essentially kamikaze robot boats packed with explosives. These are relatively easy to build—they're just regular skiffs rigged up with GPS and remote control. It's a lot cheaper than building your own navy.

Worse, the Houthis have been getting friendly with Somali pirates. And so, DARPA reckons, it's just a matter of time before Somali pirates start using the Houthi model. Rather than risking life and limb scaling the side of a ship, all they need to do is show up with a bomb-laden USV. They contact the crew of the cargo ship, and explain that unless a $XXX,XXX ransom is delivered, they'll blow the ship up.

DARPA has pre-emptively developed a concept to defend against this: The Pulling Guard. This is a small, unmanned, armed platform that cargo ships would tow behind them. In the event of trouble, the Pulling Guard autonomously sends up a quadrotor drone to get the big picture. A remote operator then views the feed, assesses the threat, and can opt to fire missiles from the Pulling Guard.

DARPA points out that all of the technology to create a Pulling Guard already exists; their focus, then, is on "marinizing" the sensors and systems, and packaging them in a modular way in order to ease manufacture. If they can pull it off, the Houthis will have to go back to the drawing board.