Answer:
- The common ground between the opposing sides.
Step-by-step explanation:
According to the question, the idea that Lincoln wishes to emphasize by using parallel structure is that the 'ground or basis for both the rival/opposing ideas are common.' He employs the parallel structure by stating that 'neither of the opposing parties was expecting the war or the duration for which it has survived and...cease' which indicates the 'terrible conditions that have been caused due to the civil war which is the product of conflicts.' Through this, he aims to signal the 'common causes of the war' which none of the parties have expected and questions God's will in this.