Japan's Navy was depleted and the islands in ruin when we nuked them, so in the case of tit for tat warfare that you prescribe the last resort wasn't needed.
But they were still capable of fending off a ground invasion and willing to do so would have cost millions of lives in the end.
But in your last resort scenario there is no need to invade, the threat had been defeated. They no longer posed a threat. Besides, boots on the ground kicking ass would have made them decide not to ever attack again right?
Like I said, a ground war would have cost us dearly in US soldiers' lives. That's why Truman decided to use the bomb. He wanted to end the Japanese agression and take Japan, quite honestly, before the Soviets attempted it.
But if you are using force based on need, there was no need to take Japan by troops or bomb, they were defeated economically and militarily. So you point about need is moot.
