join up
phrasal verb with join verb uk/dʒɔɪn/ us/dʒɔɪn/
参军，入伍
If you join up, you become a member of one of the armed forces.
"Have you been in the army for a long time?" "I joined up as soon as I'd left school." “你参军很长时间了吗？”“我中学一毕业就参军了。”