3 sentences

1. 这支主力军在关键时刻发起了进攻。

   zhè zhī zhǔ​lì​jūn zài guān​jiàn shí​kè fā​qǐ le jìn​gōng.

   This main force launched an offensive at the critical moment.

   Source: Dong Chinese Dictionary

2. 主力军在正面战场上击退了敌人的进攻。

   zhǔ​lì​jūn zài zhèng​miàn zhàn​chǎng shàng jī​tuì le dí​rén de jìn​gōng.

   The main forces repelled the enemy's attack on the front line.

   Source: Dong Chinese Dictionary

3. 中小企业是国家经济发展的主力军。

   Zhōng xiǎo qǐ​yè shì guó​jiā jīng​jì fā​zhǎn de zhǔ​lì​jūn.

   Small and medium-sized enterprises are the main driving force of the country's economic development.

   Source: Dong Chinese Dictionary