Operation Greenhouse was the fifth American nuclear test series, the second conducted in 1951 and the first to test principles that would lead to developing thermonuclear weapons (hydrogen bombs). Conducted at the new Pacific Proving Ground, all of the devices were mounted in large steel towers, to simulate air bursts.

Greenhouse represented new and aggressive designs for nuclear weapons. The main idea was to reduce the size, weight, and most importantly, reduce the amount of fissile material necessary for nuclear weapons, while increasing the destructive power. With the Soviet Union's first nuclear test just a year and half earlier, the United States had begun stockpiling the new designs before they were actually proven. Thus the success of Greenhouse was vital before the development of thermonuclear weapons could continue.

A number of target buildings, including bunkers, homes and factories were built on Mujinkarikku Island as test subjects.

The "George" explosion was the world's first thermonuclear burn, though it was just a test device, unsuitable for weapons. The vast majority of the power of George was from fission; the energy output from fusion was insignificant in comparison. (The first full thermonuclear device would be set off in the Ivy Mike explosion.) "Item" was the first boosted fission weapon, nearly doubling the normal yield of a similar non-boosted weapon.