This is a theory I have to explain the apparent BoS presence in Fallout 4. It's like 90% speculation, so take it at face value.

First off, I'm making three assumptions; The the BoS in Fallout 4 is from the CW, as implied by the name of the crashing ship being the name of King Arthur's ship, that the Lone Wanderer told Lyon about the Super Mutant's home in vault 87, and that the 200 years bit was a generalization.

So we know Arthur is meant to take leadership of the BoS. But how and to what point? The CW chapter has all but been excommunicated due to Lyon's actions, and even then the BoS is not doing well on the west coast due to the conflicts with the NCR. Basically, Arthur is a destined king of a dying empire (as it were). I believe that his rise to power isn't a good one. He get's older and starts to question Lyon's actions and abandonment of his ancestors codex. Eventually he makes his move to become leader but finds little support and Lyon's doesn't want to step down as he feels Arthur will undo all Lyons had done (help the CW). With so many local recruits, Arthur doesn't find many who will support his claim.

He leaves for the Outcasts, basically true BoS in all but name (maybe some of Lyon's flock goes with him as they still support the old brotherhood). Arthur promises to restore the eastern brotherhood and reunite them with the west coast. The outcasts lead an assault on the citadel and Arthur becomes the leader of a united CWBoS, with Lyon and his loyalists exiled. NV mentions that there was a civil war in an eastern chapter, the the outcast leaving wasn't really a civil war in that there wasn't really any fighting when they left. NV is set a few years later so this may have been when Arthur attacked in a proper civil war?

With the salvaged Enclave tech (like the Rockland Relay), he contacts the MWBoS and they form something of an alliance and share knowledge (MW gets reverse engineered Enclave tech and CW gets airships). Arthur knows he needs to expand his territory, so he sends scouts to find a good target.

At some point he learns of MIT and their synth revolution problems (fallout 3 hints that more and more synth's are seeking freedom). He knows of their advanced tech and sees this as an opportunity to strike while they're distracted. With the MW airships and the Enclave tech, they launch an invasion.

I think in the game, Beth will do what Obsidian did with NV and say that the army is small because they're involved with engagements all over, not just in Boston. Maybe the MWBoS learns of the CW plans and doesn't like the idea of them getting tech that powerful, so they attack them over it?

Any thoughts?

I think Arthur Maxson was at a very impressionable stage in his life in FO3, and Lyons most likely shaped him to be nowhere near that reactionary. If not Owyn, then Sarah (who he has a precocious crush on from what I remember).

Heck, add 12 or so years past FO3 and he could be late 20s, the age that Sarah was, and he could be in charge of the expedition to Massachusetts.

This is what I was thinking.

I doubt Arthur would become like that after being around Owen and Sarah, and I suspect the BoS in Fo4 may have some secret motive that makes them appear to be the badguys, only for them to turn out to be trying to do the right thing.

Kids grow-up. Teens get rebellious. When you were a kid, you thought your parents (or in this case guardian) was always right. Then you grow-up and start to think for yourself.

I wouldn't doubt that Arthur would question Lyon's direction for the BoS. His family founded the order, what right does Lyon have to counter their orders and vision?

If he tried to confront Lyon on some decision, and Lyons ignored him...well as a teen in that situation, I know I'd feel resentful.

Arthur was meant to lead the BoS, so he may have just seen the Outcasts as the REAL BoS and did what every kid thinks about at one point - run away from home. Only he ran to an order that actually treated him like the future leader.

If if if if if if if if.

A lot of "ifs" that I really don't see coming together into a perfect storm of reactionism in little Arthur's maturing mind.

I don't doubt that he'd question the Lyons either. But I do seriously doubt that either Lyons would brush him off. If they want him to give legitimacy to their philosophy of the order's mission, they'd give Arthur, the latest scion of the order's founder, guidance to see things as they do.

The Outcasts will die slowly, just like the Bros out west. They are insular and xenophobic, they haven't endeared themselves at all to the wasteland populace by tackling the super mutants, kicking the Enclave in the balls, and providing pure rad-free water to the people, they haven't just scored freakin' vertibirds amongst a huge pile of other victory spoils, they apparently carried over their mutiny problem into their own ranks, and they have a crummy little office building instead of the Pentagon as their main base. If Arthur doesn't turn into an angry myopic young man, he'd see the slow lingering death the Outcasts will have.

And if Arthur really does have such arrogance of "My family founded the Bros, what right does he have", then he truly does not deserve such a role.

If this was an Obsidian game maybe these cool plot twists and back stories would occur.

BUT this is Bioware. (My bad) Bethesda

Deep down you know that King Arthur is going to lead the white knights of the capital wastes into Boston to fight the evil Institute and free the wasteland. You will be leading the charge of the reformed knights of the roundtable wearing your super awesome power armor and fighting "the good fight".

We all want the depth of "Blade Runner" but we are going to get "Expendable 3" At this point I'd be happy enough with a shallow action flick like "Furious 7"

Bioware? Am I missing something? Is that a joke about the new convo system?

Maybe. In any case I'll be siding with whoever is against the BoS. Gameplay shows them as a possible enemy and I intend to capitalize on that.

For the Fallen. For the President. For the Enclave.

It's entirely plausible that he leaves Lyons back in DC, to do whatever with his ideals, and moves on to continue the prime directive of the BoS.
Once word of the institute spread through the BoS, seeing as how Rivet City scientists worked with BoS on LP and PP, I can easily see word of Dr. Zimmer from the institute either being gunned down by Harkness, or capturing the robot he's looking for, who happens to be Harkness help the BoS connect the dots on all of the sweet, sweet tech they know they want.

If everything plays out the way I imagine with CWBoS, they're recruiting wastelanders from the DC area. If this is the case, all the more reason to leave a base in DC and simply expand.

Sums it up perfectly.

If obsidian wrote another piece of garbage we'd get 3 factions sending us to do the same 3 things, with different quest names. Their idea of plot twists are having to kill the president in this version of the same quest, and the 200+ year old man is unsurprisingly a vegetable!

At least the killing of BoS in the trailer gives us an idea that they may be a faction you can oppose.

Thankfully the better company is at the helm.

I am a bit confused. I was under the impression that the West Coast Branch of the Brotherhood was not aware of the actions of Lyon's part of the Brotherhood. Im sure in FO3 there is a comment that they had no way of getting in communcation with their home base.