I can only give you an outline, as giving you a whole essay takes too much of my time and it does not help you to improve your essay writing skill
1. (Intro)
1) what the atomic bomb does(physically and mentally)
2)the situation(heavy casualty in the battle of Iwo Jima, a island close to Japanese mainland, high casualty estimate for operation downfall, soviets gathering forces at Sino-soviet boarder preparing to launch assault and capture land)
2. (Body 1)What is the strategic goal of US(unconditional surrender of Japan, as soon as possible, because the Japan must not fall into the hand of the soviets, that goes with Korea, too)
3. (Body 2)What are alternatives other than atomic bombs(1000plane air raids(explain), naval blockage(explain))
4. (Body 3)What is the optimal option(naval blockage, more air raids or atomic bomb)
5. (Conclusion)Summarize
They served with the Confederacy because they believed that the Confederacy fought the big central government that was being imposed by the Union and they wanted to fight this so as to keep sovereignty over their own land. They believed that the Union government didn't care about them while the Confederacy would give them room to breathe.