System(s) Changes:
Added Frontlines: There are now frontlines that get generated for each country (and the fronts of the country's allies/vassals/unions for reinforcement). When the player shift-clicks a frontline tile with units selected, those units are allocated to that frontline. When the player control-clicks a frontline tile, the frontline's attack mode will be toggled on/off. An animation on the frontline plays to indicate the current attack mode. Frontlines advance in layers and the AI also attacks/defends via frontlines.
Known Bugs/Issues:
When a frontline advance causes one frontline to split into two or three, some units get unassigned due to frontline succession issues. The player has to manually reassign units to frontlines. The AI struggles to reassign units properly.
When a frontline gets confusing and it's all over the place, units get confused and simply freeze, doing nothing. This requires the player to manually move units to resolve the issues. Simply reassigning the units to a frontline does not work. This is particularly extreme in larger fronts (Germany vs. Russia) but less prevalent in smaller fronts (Germany vs. Belgium).
Sometimes, when all of the units of a front are pinned and preoccupied with a battle, another unit just starts capturing tiles behind enemy lines and it just doesn't make any sense. I believe this bug is present in the stable build as well, but it's less prevalent there.
Encircled units that are cut off from the frontline/mainland don't know what to do, so they just stand there and do nothing.
Rationale behind Unstable Branching: This patch is in the unstable branch and this update does not affect the default branch due to the bugs introduced by the new frontline system. I'll have to spend a while working on this system to get it to work properly.