YOU SAID:

those who survive long enough on the battlefield start to think they're invincible

INTO JAPANESE

戦場で十分長く生き延びた人は、彼らが無敵だと思うようになります

BACK INTO ENGLISH

Those who have survived long enough on the battlefield will think they are invincible

INTO JAPANESE

戦場で十分長く生き延びた人は、彼らが無敵だと思うでしょう

BACK INTO ENGLISH

Those who have survived long enough on the battlefield will think they are invincible