It is a common misconception of childhood; this imaginary lion.
The story as I heard it:
A third grader takes a geography quiz. In answer to the question “What is the equator?” he wrote:
“The equator is an imaginary lion running around world.”
Absent any posted signage indicative of their existence, borders between states and nations are entirely imaginary lines. Borders exist only on paper and in the popular imagination. A convention is an instance wherein a group of people come together and agree to a principle. They agree that said border exists. They agree that rules exist that govern that imaginary border. They agree to abide by self-imposed limitations to their own actions respecting this imaginary line. If they come to disagree, and too many exceptions are made, this imaginary line simply dissolves.
When one country invades another country’s territory this border no longer exists in the popular imagination. The dissolution of this imaginary line in the minds of some (if not all) of the people always precedes war. World Wars I and II are instances wherein most (but not all) borders simply (and hysterically) dissolved in the popular mind.
To maintain a border it is incumbent upon both of the governments (and also the people) of (any two) contiguous countries to respect and protect this otherwise imaginary line.
There is the familiar apothegm:
“Good fences make good neighbors.”
This applies not just to individuals but also applies to countries as well because countries are made up of individuals.
To some, this imaginary (border) line doesn’t matter. (It is immaterial; therefore it does not exist.)
To others, it is an impediment to their actions and should not exist.
To still others, it is both a fence and a defense against trespass and encroachment.
When this imaginary line dissolves as a principle in the popular mind it ceases to exist in reality.