Close to the end of the war, Japan – which had made pretty extensive use of biological weapons against China – was working on also hitting the US with biological weapons. We were far enough away that it would have been difficult, but where they had been able to employ biologicals, in Asia, they did.
Operation PX, also known as Operation Cherry Blossoms at Night, was a planned Japanese military attack on civilians in the United States using biological weapons, devised during World War II. The proposal was for Imperial Japanese Navy submarines to launch seaplanes that would deliver weaponized bubonic plague, developed by Unit 731 of the Imperial Japanese Army, to the West Coast of the United States.
That being said, Japan wasn’t even the expected target of the Manhattan Project. Germany would have been, but was defeated via conventional force prior to the project reaching completion.
That’s one of the risks of kicking off a war.
Close to the end of the war, Japan – which had made pretty extensive use of biological weapons against China – was working on also hitting the US with biological weapons. We were far enough away that it would have been difficult, but where they had been able to employ biologicals, in Asia, they did.
https://en.wikipedia.org/wiki/Operation_PX
That being said, Japan wasn’t even the expected target of the Manhattan Project. Germany would have been, but was defeated via conventional force prior to the project reaching completion.