The US didn’t have dozens more in reserve, there was only one more working build available if I recall correctly.
More generally, the US and Japan had been in a state of total war for nearly four years at that point and many other Japanese cities had already been razed by conventional bombing runs. Tokyo was completely destroyed in march of 1945 with similar casualties.
It was well past the point of warning shots, the two nations were already committed to obliterating eachother.
Latest Answers