Precipice of War

Precipice of War is a Railroad quest in Fallout 4.

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

Bugs
Another way to fix it (Not certain) is to walk around and/or back and fourth below the Vertibird, this may cause it to crash.
 * 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 your primary gear features this prefix.