Plaque is the main enemy of healthy teeth.

牙菌斑是牙齿健康的首要敌人。

youdao

Unite with all the forces that can be united and strike blows at the main enemy.

团结一切可以团结的力量，打击主要敌人。

youdao

The main force of the enemy met with a check.

敌人的主力受到了牵制。

《新英汉大辞典》

America promoted a strong dollar throughout the 1990s, when inflation was still thought to be the main enemy.

ECONOMIST: Currencies

He says that Russian people once again see the United States as the country's main enemy.

NPR: Rice, Gates Appeal to Moscow for Missile Defense