What most people don't get is the true purpose of theEdited: The Schengen area:
#No delaystreaty was mostly for the benefit of EU citizens crossing internal borders, and it lets them (and others) cross internal borders without systematic controls.
To make that happenthis feasible, there should be no or few controls forit was necessary to create an anyoneunified system on thosefor short-stay visa and visa-free entry. This does not mean that everybody within the Schengen area is allowed to cross internal borders at will, because it is a bit pointless to show an ID cardor to provecross them without any documents in all cases. But it was deemed that you do not have to show a passportthe potential for abuse was acceptable for the convenience of the citizens, who can now drive through the smaller countries without once halting their cars.
- It is not true that anyone within the Schengen area is allowed to cross any internal border. But generally, those cases are few enough, and unimportant enough, that this is not routinely controlled.
- A visitor with a national long-stay visa may only visit the other Schengen states for 90 out of 180 days.
- A visitor with a short-stay visa may be restricted to only part of the Schengen area.
- A refugee may be under even more restrictions where he can stay.
- It is not true that you can cross internal borders without documents. You have to bring them, even if there is no border checkpoint, and present them to the authorities on demand. (Details on who can ask where are covered by national law.)
In exceptional cases,The reintroduction of internal controls may be reintroduced. That doesmeans possible delays at the internal borders (for EU citizens and visitors alike), and increased administrative overhead for those EU citizens whose countries did not change who can travel whererequire IDs before. Internal controls in any waydon't abolish the unified visa system. It does not even change, just the requirements which documents one must carry. All it changes isfrequency of checks along the likelihood that those documents will be checkedway.