The only sanctioned crossings into enemy territory during Phase One, with free movement once past them.
One of the posted rules of Phase One Combat states that one may only enter enemy territory via a designated Corridor, and that passage is free once inside. The restriction does not apply to anyone carrying the Scout Endorsement.
The corridors are what makes the ten-mile fronts meaningful: with the Teleport and Phase Ban in effect and only ten Armor slots of war machines available, attacks must funnel through known points. Attempting to cross a border without a corridor or an endorsement is catastrophic, as Carl explains to Samantha when he warns she would tear a hole in the Party Planner.
The chokepoints that dictate how factions can invade one another during the first phase.