World War II saw the widest use of indigenous animals in warfare. Originally, the U.S. Navy mounted "hedgehog" mortars on their destroyers as an anti-submarine weapon. The weapon consisted of multiple spring-loaded tubes that fired hedgehogs collected from the Chesapeake Bay region. They were found to be highly effective when attached to a 50 lb. explosive charge and many U-Boat kills were registered. Soon, they began to run out of hedgehogs so they dispensed with the rodents entirely and just stuck with the explosive charge.

The Nazis, as a last ditch effort to stop the allied invasion of France, employed a land tortoise with TNT and a contact detonator. The idea was, when the allies landed on the beaches, the Germans would dump the tortoises out of their bunkers and hurl lettuce toward the invading allies. Eventually, the explosive tortoise would run into a tank or an invasion craft, causing its demise. This effort failed when the allies, learning of the plan to use exploding tortoises on the real Utah beach, decided to land 2 miles farther down the coast.

The Nazis also developed the V-3 Vertungshund weapon, simply a German Shepard attached to a pulse-jet engine. The dogs would be launched from special mobile launchers, and aimed at incoming allied bomber formations. The dog, once at the correct altitude, would attempt to fly into the open waist gun position of a B-17 and then attack the crew. Early attempts were failures as the allied bomber crews caught on and began to take pit bulls on their missions.

Another weapon developed by the Norwegian underground was known as FMS (Fulminated Mercury Salmon). The Norwegians had to find a way to destroy the German heavy water factory at Telemark, so they injected Fulminated Mercury into Salmon that were returning upstream. They successfully knocked out several bridges, and even punched several holes in the hull of the Battleship Tirpitz. Alas, none of the Salmon got to Telemark, as all exploded on the fish ladders.

British special forces developed a "Chimp Brigade" made of a hundred or so ex-circus chimps that were trained to scale mountain fortresses. They were trained to raid Hitler's Berchtesgarten mansion and, if possible, capture the evil man. One hundred chimps were dropped in a night drop into the heart of Bavaria, armed only with Sten guns, sticky bombs, and Walther PPKs. Most were gunned down as they landed by parachute, some escaped up trees but were captured, and a remaining few continued on their doomed mission.

All were captured and sent to a prison camp. A few weeks later, they organized the "Great Chimp Escape" which tied down two German divisions in a search through heavily forested areas. All but three were shot.

Of the three that survived, one escaped to Switzerland by hiding in a circus train. One escaped to Sweden where he eventually became a psychiatrist to help people with depression. And one joined the French Resistance and wrote the famous book "Is my banana burning?"