What determines where armies can move?

As expected, my troops can move into territory that:
- Belongs to me
- Is occupied by me
- Belongs to a nation that grants me military access
- Belongs to a nation I am at war with
However, in some cases I have noticed that my armies can walk over foreign territory when I have no diplomatic relation with the owner. For instance as France, if I fight wars with Denmark or Southern Italy, I am sometimes able to walk my troops all the way through 5 or so different nations, most of them neither allied with me nor otherwise granting access rights. This usually creates a "you have troops in foreign territory" message in the top left.
I also notice that sometimes foreign armies walk over my territory, even though again I have no relation with them and never gave them access. For instance, right around 1444 as Spain, sometimes Genoa will just waltz right through Spain.
Why are armies sometimes able to enter foreign territory without access rights? What is the full of list of conditions that allow a troop to enter a given territory?
Best Answer
There has been some controversy over this phenomenon in the recent past. You used to need military access granted to you directly to be able to enter neutral territory, but this was open to exploitation, such as hiding units in neutral territory that your enemy can not pass.
The developers chose to implement a change, meaning that from version 1.11 onwards all sides in a war can pass through the territory of a nation, as long as they grant military access to one or more countries involved in the war.
Given the fact that many AI nations use up the majority of their Diplomatic relation quota on access treaties this can lead to a very open playing field indeed and vastly complicates getting military victories when playing as a smaller nation.
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Answer 2
In addition to the change in patch 1.11 as mentioned by Richard, there is a state called "Exile" which is when your troops are fighting a war in a foreign nation and then the war ends without your troops pulling out. This puts them in a state where they do not have to respect military access, but cannot fight until put into friendly territory. Once they go into friendly territory, they lose exile and must respect military access.
Answer 3
In addition to what is listed by others, if the Holy Roman Emperor is in a war, both sides can enter any province within the Holy Roman Empire (whether or not it belongs to a prince of the HRE), and access is allowed between subjects and vassals.
For completeness, I will summarize the other answers:
Your troops can move into territory that:
- belongs to you
- you have military access to
- is part of the Holy Roman Empire, if you are in a war with the Emperor
- belongs to your subject or overlord [*]
- your enemy or ally in a war can move into
and if your troops are exiled, they can move into any land province.
[*] I'm not sure about the exact details, here: can anyone confirm? The wiki says that a protectorate has access to its overlord; I assume that's the case for other subjects, and that the overlord has access to all its subjects.
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
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