All armies, garrisons, and cities control the territory around them. All infrastructure and industries within this zone of control change ownership accordingly. The zones of control are called Frontlines in the game. To see the dynamically changing frontlines, select the Map Filters (1) when in the max zoomed out view and tick Fronthnes (2) The Frontlines are drawn on the map (3) and do not follow any political lines like borders, except when the nations in question are in peace. When armies move, frontlines are updated accordingly.
Armies can capture cities. When an army (4) is near an enemy city without any defenders nearby, the military will capture the city. The time it takes depends on the size of the city and the capturing army, and the progress is shown in a bar (5) above the city. When the bar is complete, the city changes hands, and the new owner will receive all the terrain controlled by the city. States with low support will not change loyalty even if the enemy entirely controls them. In this case, the state is considered occupied. Losing cities to the enemy will lower support in the states and lower National Morale when the news reaches the public.
When IIPs are captured, their transport capacity will decrease to zero for a short time, as the logistical infrastructure there is destroyed by the retreating previous owner.
The condition will recover shortly, but before this happens, the infrastructure will not be able to carry supplies effectively, leading to supply shortages if advancing deep into enemy territory without consolidating and constructing depots. The IIPs’ condition can be seen in the tooltips.