‘And he said, “No, but as Captain of the host of YHWH am I now come.” '
To Joshua's astonishment the man replied that He had come as Captain of YHWH's host. At the mention of YHWH's host Joshua's mind may well have gone back to the ‘ten thousands of holy ones' described by Moses (Deuteronomy 33:2). So he may have seen this Man as having come, with YHWH's hosts backing Him, to fight alongside Israel and bring God's judgment on Jericho, with Jericho being seen as representative of all the Canaanites, because their iniquity was now full (compare Genesis 15:16). It indicates that the cry of Canaan's deep sinfulness, with its distorted religion, sexual perversions and child sacrifices, had reached to heaven.
We can compare this to some extent with Elisha who was also surrounded by the invisible host of YHWH (2 Kings 6:17), and his vision of the chariots of God who were there to fight on behalf of Israel, the ones who were Israel's true chariots (2 Kings 2:12). In both cases the idea was of God's power behind His chosen servant. Compare also the angels of God who met with Jacob on his return to Canaan, ‘God's host' (Genesis 32:1-2). But the drawn sword stresses that here the emphasis was on judgment rather than protection.
Alternately, however, and possibly preferably if not as spectacularly, we may see ‘the host of YHWH' as referring to ‘My hosts, My people, the children of Israel' (Exodus 7:4) with the idea being that this man was claiming to be their supreme general, indicating that He was therefore Joshua's superior officer, the Captain of the host of Israel, ‘the Angel of YHWH' in contrast with Joshua, ‘the servant of YHWH', and that He had come to lead them with the sword of judgment already drawn. In this case we have the picture of Israel as YHWH's avenging host under YHWH's direct command who must now bring judgment on Canaan for its evil ways.
Joshua 5:14 b.
“And Joshua fell on his face to the earth, and performed worship, and said to him, “What does my Lord say to his servant?”.'
Compare Balaam's response to the Angel of YHWH, ‘he bowed his head and fell on his face' (Numbers 22:31). Joshua's act of worship demonstrated that he now knew that this was the Angel of YHWH, YHWH Himself revealed in human form. He was aware of His numinous presence, and, filled with awe, he yielded to Him in total submission. “What does my Lord say to his servant?”