It seems from the fact that this “commander of the Lord’s army” accepted Joshua’s worship (unlike the angel in Revelation 19:10; 22:8-9) and that his words mirrored God’s words to Moses in Exodus 3:5, that this commander was actually Jesus.
When Joshua found out who he was, he instantly dropped his “Are you with us or with our enemy” question, and said, “What does my Lord want to say to his servant?”
That’s the attitude we should take with our Lord. Not questioning whether he is on our side or not, or demanding he do as we ask.
But rather coming before him humbly, and saying, “What does my Lord want to say to his servant.”
Lord, I come before you now. I make no demands. I don’t question your love or loyalty to me. I simply kneel quietly before you today, and say, “What does my Lord want to say to his servant.”
Moment to Moment (Bible blog) 