Precipice of War

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

Quick walkthrough

 * Warn Desdemona
 * Defend Railroad HQ
 * Talk with Desdemona inside HQ
 * Secure the Church
 * Talk with Desdemona now located towards the exit of the church
 * Meet Tinker Tom at the Brotherhood Police station
 * Eliminate all present Brotherhood forces
 * Talk with Tinker Tom
 * Defend the Vertibird
 * Talk to Deacon

Detailed walkthrough
''Note: This mission will have you do battle with a lot of enemies using energy weapons, and almost a dozen Brotherhood Soldiers in Power Armor. Make sure you are geared appropriately. ''

Railroad HQ

The Brotherhood of Steel sends a unit to assault Railroad HQ. They will begin their attack by coming from the escape tunnel. Join the Railroad in the battle, and kill them. After they are dead, speak with Desdemona for an update on the situation. She will inform you that the catacombs is the only way out. She instructs you to join Glory in the battle, who already went ahead.

As you exit HQ and into the catacombs, you will find Glory at her dying breath. (Deacon approves if you tell Glory she'll make it) Regardless, the heavy falls victim to her wounds. It is up for you to secure passage.

Catacombs / Old North Church

You must now continue your journey through the catacombs and make your way towards the main floor of Old North Church. As you make your way through, the secret wall will burst open, revealing a Brotherhood Knight in power armor, and another knight in standard BOS Combat Armor. Kill them, and then continue making your way through the crypt. More Initates and Knights will be waiting for you, and some will be wearing power armor. Kill anyone along your way towards the main floor of the church.

Once you reach the main floor, another handful of Brotherhood Initates, Knights, and Aspirants will be waiting for you. They all need to be eliminated.

''NOTE: Depending on how you completed The Lost Patrol, Paladin Brandis could be among them. He will be equipped with either a minigun or a gatling laser (randomly generated), and has a higher amount of hit points compared to the others soldiers. He is considered a dangerous enemy. After you kill him, you have a chance at grabbing the ultra rare jet black version of the BOS officer uniform should you choose too.''

Once all the Brotherhood soldiers on the main floor are dead, Desdemona and the rest of the Railroad will make their way upstairs to join you. After grieving over the loss of Glory, she states that the Brotherhood underestimated the Railroad, and that their next attack will far worse. So she explains that the next step is to eliminate the Brotherhood threat as a whole: by destroying their flying fortress, the Prydwen. She then tells Tom that operation "Red Glare" is a go. However, he explains that it requires a Brotherhood Vertibird. Desdemona informs you that there is one on the Cambridge Police Station, and sends both you and Tom there.

Cambridge Police Station

Warning: ''When meeting up with Tom, it's not recommended to fast travel directly to the station itself, as you will end up spawning with exposed cover right in front of the Brotherhood soldiers, all of which are hostile. College Square, which is right around the corner of the station, is a safer area to fast travel. Either way, you have to meet up with Tom before proceeding.''

Outside of the station, after Tom finishes with his paranoia episode of the Institute possibly watching their every move, he explains that the Vertibird is on the roof as promised. He also says that the station needs to be cleared out of enemies.

At least two enemies will be in Power Armor. The rest will be in their standard Brotherhood uniforms such as Brotherhood Fatigues or Field Scribe's Armor. You need to kill every one of them outside and inside the station. Be sure to check the garage as there may be 2 inside. Note that the garage door may be closed, so you will have to enter it from inside the station. Once they are all dead, make your way to the roof.

Tom explains to you that the Prydwen remains airborne because of bags full of hydrogen. He gives you an explosive charge and requests you to use it to destroy those bags, to return to the Vertibird to watch the "crash landing" from a distance. Afterwards, he proceeds to prep the vertibird for take-off.

While Tom preps, an enemy vertibird will arrive at the scene and attack. Kill everyone inside, or shoot down the Vertibird. Once they're no longer a threat, talk with Deacon to complete the quest. The quest Rockets' Red Glare will then begin.

Bugs
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.