Backpack Nukes?

 H-912 transport container for Mk-54 SADM

The U.S. had them in its arsenal:

The Special Atomic Demolition Munition (SADM) was a family of man-portable nuclear weapons fielded by the US military in the 1960s, but never used in actual combat. The US Army planned to use the weapons in Europe in the event of a Soviet invasion. US Army Engineers would use the weapon to irradiate, destroy, and deny key routes of communication through limited terrain such as the Fulda Gap. Troops were trained to parachute into Soviet-occupied western Europe with the SADM and destroy power plants, bridges, and dams.
The project, which involved a small nuclear weapon, was designed to allow one person to parachute from any type of aircraft carrying the weapon package and place it in a harbor or other strategic location that could be accessed from the sea. Another parachutist without a weapon package would follow the first to provide support as needed.
The two-person team would place the weapon package in the target location, set the timer, and swim out into the ocean, where they would be retrieved by a submarine or a high-speed surface water craft.
In the 1950s and 1960s, the United States developed several different types of lightweight nuclear device. The smallest of these was the W54 warhead, which had a 10.75 inch diameter (270 mm), was about 15.7 inches long (400 mm), and weighed approximately 23 kg (50 lbs). It was fired by a mechanical timer and had a variable yield equivalent to between 10 tons and 1 kiloton of TNT. The W54 nuclear device was used in the Davy Crockett Weapon System.
The Atomic Demolitions Munitions school was located at the U.S. Army Engineer Center on Ft. Belvoir, Virginia, until it was closed in 1985.

I'm still amazed we didn't destroy the entire world during the Cold War.  Not that we didn't come close a bunch of times.