The world's first military submarine

 The world's first military submarine used in combat was the Turtle, designed by American inventor David Bushnell in 1776. This vessel was used by American colonial forces to attack British warships during the American War of Independence.

Here's a timeline of the evolution of military submarines over time to get a more complete picture:

The First Military Submarines in History

The Turtle (1776): Uniquely shaped like a walnut or egg, this vessel was manually propelled using paddles and designed for a single crew. Its mission was to secretly attach explosives to the hulls of enemy ships underwater.

It was launched on September 7, 1776. The target was the British warship HMS Eagle. The mission failed due to the inability to attach the explosives to the hull.

The Turtle

Nautilus (1800): Designed by Robert Fulton for the French Navy, it was one of the first submarines to use an internal steering system and carry advanced explosives.

Nautilus

The First Modern Military Submarine

If the benchmark is a modern military submarine officially adopted into a structured naval fleet, then that status is held by:

USS Holland (SS-1)

USS Holland (SS-1): Officially commissioned by the United States Navy on October 12, 1900. Designed by John Philip Holland, this vessel was the world's first modern military submarine, combining an internal combustion engine (gasoline) for surface propulsion and a battery-powered electric motor for submersion. This submarine became the forerunner of the design of today's global submarine fleet.