Precipice of War

Precipice of War is a Railroad main quest in Fallout 4. It is the follow-up from Underground Undercover and is part of a chain of quests, all of which are tied together, to an alternative method of completing End of the Line.

Detailed walkthrough
Upon warning Desdemona of the Brotherhood of Steel's impending attack, it begins immediately upon delivering the warning. Defeat the Brotherhood soldiers that attack from the route leading to the escape tunnel. After they are dead, speak with Desdemona, who informs the Sole Survivor that the catacombs are the only way to escape. Glory has already gone ahead to secure passage, and Desdemona asks the player character to assist.

Entering the catacombs, Glory has been grievously wounded. Regardless of how the player character answers her, she succumbs to her wounds. Two knights— one in power armor, the other in BOS combat armor— will burst through the sealed entryway with explosives. Kill them and then continue back through the crypt, fighting initiates and knights along the way. Up on the main floor of the church, another unit of Brotherhood initiates, knights and aspirants will be waiting. They will all need to be eliminated.

''Note: Paladin Brandis may be among the enemies here, if he was convinced to return to the Brotherhood during The Lost Patrol. He has unique gear which can be obtained by shooting out the fusion core on his power armor, but he is armed with a minigun and is considerably more dangerous than the average enemy here.''

Once all of the soldiers are dead, Desdemona and the others will enter from the catacombs. Grieving the loss of Glory, they note that the Brotherhood underestimated the Railroad's strength and will attack again with a much greater force, thus the Old North Church will not be safe until they are eliminated as a whole. Operation "Red Glare," the destruction of the Prydwen, is a go. To carry it out, the player character must steal a vertibird from the Cambridge Police Station. Head there immediately and rendezvous with Tinker Tom.

Outside of the station, Tom mentions that the vertibird is still on the roof and all Brotherhood personnel must be eliminated (including Haylen and Rhys). Enter the station and open fire, making sure to check every room, including the garage, which might be closed from the outside. Once they are all dead, head to the roof and kill the remaining soldiers. Tom hands over an explosive charge to affix to the tanks of hydrogen aboard the Prydwen, which will detonate and ignite the hydrogen in the envelope, bringing the airship crashing down.

While Tom preps the vertibird, an enemy vertibird will arrive and attack. Shoot it down or kill everyone aboard, then talk with Deacon to complete the quest. The quest Rockets' Red Glare will then begin.

Bugs

 * Tinker Tom will be sitting on the ground after taking too much damage and the quest will not progress unless he goes to the Vertibird. It will say "Defend the Vertibird" but Tinker Tom is bugged and so the attacking Vertibird will not appear to advance the quest. This can be fixed by fast traveling to another location, and then back again.
 * Shooting the attacking Vertibird with a weapon that can cripple an enemy's legs thanks to its unique prefix will cause the Vertibird to become indestructible - its health can be depleted but it won't crash, thus preventing the mission from progressing to the next stage. Make sure to bring a powerful backup weapon if the primary gear features this prefix.
 * Another way to fix it is to walk around and/or back and fourth below the Vertibird, this may cause it to crash.
 * Even if you shoot down the Vertibird, the quest can become bugged and not progress past "Defend the Vertibird". Entering with the enemy Vertibird targeted will progress the quest correctly.
 * The church may be impossible to secure even after killing all the Brotherhood members inside.
 * The setstage command also does not seem to fix this.
 * At Cambridge Police Station, killing some members of the Brotherhood of Steel will count towards your Murder statistic in your Pip-Boy, despite them clearly being hostile towards the player.
 * If the Brotherhood didn't become hostile during Mass Fusion, either by activating the fast travel point manually and thus skipping the confrontation or by using the console to bypass that stage of the quest, there will be at least one Brotherhood soldier in the church friendly to the Sole Survivor and fighting against the other Brotherhood soldiers. Killing them will make the quest progress and will not count as murder in your Pip-Boy.