major first flank

例句：

1. 1. The major first flank of the battlefield was heavily contested.
   战场的主要第一侧面被激烈争夺。
2. 2. The primary front in the conflict shifted eastward.
   冲突中的主要前线向东移动。
3. 3. The frontier between the two countries is still unsettled.
   两国之间的边界仍然未定。