The US mainland was bombed 1,000 times by airborne bombs during World War II. Which country was responsible for these bombings?

Japan

Among the lesser known weapons used in World War II were airborne balloon bombs. Nearly ten thousand balloons were launched from Japan.

(They also twice used submarine-launched aircraft to bomb US mainland.)

They were the only successful bombing attacks on the mainland of United States. School children in Japan were asked to make gigantic balloons, thirty-three feet in diameter. The balloons, when filled with hydrogen gas, were buoyant enough to carry thirty-three pounds of bombs, as well as a few incendiary bombs. The balloons could travel on air currents at up to 120 miles per hour. Out of the nearly ten thousand launched, about one thousand reached land. Although the bombs caused a few fires, American officials asked people not to talk widely about them. In 1945, Six people, five of them children, were killed at a church picnic, when they saw a deflated balloon and touched the bomb, not knowing what it was.