Zhànyì dǎxiǎng hòu, zhǔlìjūn cóng zhèngmiàn fāqǐ le jìngōng.
Once the battle began, the main force attacked head-on.
Tips
usage
The figurative sense is the common one today: the group that does most of something, as in 消费的主力军 'the main consumers' or 就业的主力军 'the main source of jobs'. Compare 生力军 'fresh troops, new blood'.
