The Danites were "seeking an inheritance" for themselves (cf. Josh. 19:47,48). In other words, they still had not inhabited the full portion of land that was to be theirs in Canaan; there was more conquering to be done. Five men of Dan are sent out as spies and they happened to lodge with Micah in route. They are impressed with Micah's priest, recognized him as a Levite by his speech, and asked counsel of him: Would their current mission be prosperous? Micah's priest replied - "Go in peace. The presence of the LORD be with you on your way" (Jud. 18:6). His advice turned out to be good, which shows us that even false prophets get it right some of the time (cf. Deut. 13:1-5). As a side note, even today people can recognize us by our speech--whether it be good or bad. Let us always take heed to our speech (cf. Matt. 12:36,37; Col. 4:6).
The spies from Dan depart and determine that the territory they desire to conquer is both good and easy to claim. The current inhabitants lacked treaties and would not be able to withstand an attack from the tribe of Dan, the spies believed. After the spies return, 600 men from Dan take up weapons and begin the journey to Laish to conquer it. On their way the group stopped at Micah's house. They greeted him and proceeded to take "the carved images, the ephod, the household idols, and the molded image" (Jud. 18:17). The priest objected, but they replied - "Be quiet, put your hand over your mouth, and come with us; be a father and a priest to us. Is it better for you to be a priest to the household of one man, or that you be a priest to a tribe and a family in Israel?" (18:19). This pleased the priest and he doesn't mind stealing from Micah in order to accept this promotion! Some preachers today are only interested in personal gain; they have more in common with this Levite than they do the Lord! "The hireling flees because he is a hireling and does not care about the sheep" (John 10:13).
"Then they turned and departed, and put the little ones, the livestock, and the goods in front of them" (18:21). This was a tactical move; they evidently expect Micah to gather some resistance against them and try to attack them from the rear. Micah does try to organize such a party but it is small and weak compared to the Danites.
"And they called out to the children of Dan. So they turned around and said to Micah, 'What ails you, that you have gathered such a company?' So he said, 'You have taken away my gods which I made, and the priest, and you have gone away. Now what more do I have? How can you say to me, "What ails you?"' And the children of Dan said to him, 'Do not let your voice be heard among us, lest angry men fall upon you, and you lose your life, with the lives of your household!' Then the children of Dan went their way. And when Micah saw that they were too strong for him, he turned and went back to his house" (18:23-26).
What a sad day for Micah the idolater! After being robbed and now threatened by this large force of men, he realizes this is a lost cause and gives up. Evidently his gods weren't worth dying for (but the true God is!). When there is no ruler in the land, the strongest do as they please and get away with it (in the temporal realm anyway). One can only wonder if Micah went home and made some more useless gods for himself!
It would have been one thing for the men of Dan to take away the items of false worship in order to destroy them, but unfortunately that was not their intent. They, after successfully conquering Laish, "set up for themselves the carved image; and Jonathan the son of Gershom, the son of Moses, and his sons were priests to the tribe of Dan until the day of the captivity of the land. So they set up for themselves Micah's carved image which he had made, all the time that the house of God was in Shiloh" (18:30,31). Like leaven, the sins of one man (if unchecked) spread to a larger group of people. The captivity referenced here is certainly not the Babylonian captivity but one of the periods of captivity prior to the reign of King David (e.g., I Sam. 13:19,20).