Final answer:
The central idea that Lincoln develops in the speech is that leaders should have considered the cost of war more carefully.
Step-by-step explanation:
The central idea that Lincoln develops in the speech is that leaders should have considered the cost of war more carefully. The longer the war continued, the more it began to affect life in both the North and the South, with increased need for manpower and ongoing challenges in keeping the war effort going. This suggests that Lincoln believed leaders should have taken into account the long-lasting and detrimental consequences of war before entering into it.