So a halfling thief uses disengage to dance past two lines of guards and attack an opponent in the back row then a bonus action to run(disengage) back through the line of guards to stand behind his own troops.
Is that legal?
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You can use disengage to do this, but disengage provides no movement on its own. You have to use your movement, it just prevents opportunity attacks. See the definition:
If you take the Disengage action, your movement doesn’t provoke opportunity attacks for the rest of the turn. (Players Basic p72).
This lets the rogue move right through enemy lines with no repercussions (provided there is ample space to move between them and they have enough movement).
This leaves the rogue likely alone and without a flanking buddy (unless they spend a turn disengaging to join and forgo an attack).
However, what you're describing is too many actions. Here's the action sequence:
If you have movement left you can then move back without repercussions, but again, since disengage provides no movement, you're capped at 30'. If you take your bonus action to disengage, you only get the 1 bonus action and can't take another to dash (you'd have to use your action, which you want to use for an attack here).
The character can move up to his Movement for free in his turn, can make one Action per turn and can do one Bonus Action per turn. Attack is an Action, and for a Thief, Disengage is either an Action or a Bonus Action. So what he did could be legal:
TL;DR it depends on whether he has enough Movement to do all that without needing a Dash action.