Fire Support Main Story, Brotherhood of Steel Quest
This quest starts automatically when you get to the area south of Lexington, close enough to College Square and Cambridge Police Station.
Related points of interest
Stages of "Fire Support" quest
1. Listen to Military Frequency AF95
While traveling near Lexington and College Square, my Pip-Boy has received a military radio broadcast. I should listen to it and find out what the broadcast is transmitting.
3. Assist the Soldiers
Kill all the ghouls attacking the post.
4. Speak to Paladin Danse
I've assisted a group of soldiers who were under attack outside of Cambridge Police Station. Now that the fight has ended, I should speak to the soldier's commander, Paladin Danse.
Talk to Paladin Danse and tell him that you want to join the Brotherhood of Steel.
5. Quest completed
After speaking with Paladin Danse, I've accepted his offer to join the recon team and lend them a hand.
or, if you refused to help ...
After speaking with Paladin Danse, I've decided not to assist them any longer. If I ever change my mind, I could always return to the Cambridge Police Station.
Next / new quest is Call to Arms (Main Story, Brotherhood of Steel).