Semper Invicta

Semper Invicta is a Brotherhood of Steel main quest and an achievement/trophy in Fallout 4.

Detailed walkthrough
Once the Survivor completes the quest Call to Arms, this one will show up automatically, asking the player character to go back to Cambridge police station and talk with Danse.

After the discussion, Danse asks the Sole Survivor to help both Rhys and Haylen doing their respective job.
 * Cleansing the Commonwealth (repeatable)
 * Quartermastery (repeatable)

Once either of the requisite quests have been done, the quest The Lost Patrol may begin after talking to the Lancer Captain.

The quest will complete once the both requisite quests have been completed. The requisite quests are repeatable whether or not this quest has been completed.

Behind the scenes
Semper invicta is Latin for "always invincible.". It is also the motto for the city of Warsaw, Poland since World War II, a testament to the strength of the city.

Semper Invicta