This quest is aquired at the Temple of the Union from their leader Hannibal. You ultimately have three tasks to complete this. First, head to the Museum of American History and locate a picture of the Lincoln Memorial building. Then head to the Memorial itself to make sure there are no mutants there. There aren't any mutants but there are a bunch of slavers. Considering Hannibal and his friends are freed slaves, there is clearly going to be a decision here. You can do a few things: either kill all the slavers immediately and report back to Hannibal, tell them where the slaves are and help them, tell them where the slaves are and fight against them, or tell them nothing and report back to Hannibal who will want to mount an attack. I would suggest just killing them on the spot. Once you give Caleb the picture and tell Hannibal the place is clear, they will set out for the Memorial. Fast travel over to the Mall Northwest Metro Station to meet them and finish the quest. You might have to wait for them to show up, so hit Back (360) or the T key (PC) to wait a few hours.
See Head of State for more specific info.