International boundaries look arbitrary on a map, and some are. Most, however, follow one of a small number of origins, and each origin creates its own characteristic problems.
Rivers move, which boundaries do not
Rivers were adopted as boundaries because they were visible, defensible and easy to describe in a treaty without surveying.
Channels shift. A river that migrates gradually usually carries the boundary with it, while one that jumps its course in a flood generally does not, under long-standing conventions.
The distinction between gradual and sudden change is legally decisive and factually contested, which is why river boundaries generate disputes centuries after they were drawn.
Mountain lines are less obvious than they look
Ranges were used because they separated populations and impeded movement, and treaties often specified the watershed as the dividing line.
Watersheds and highest ridges do not always coincide, and in glaciated terrain the divide can be genuinely ambiguous on the ground.
Where valuable resources or strategic passes sit near the line, small ambiguities become significant, which is the origin of several long-running disagreements.
Straight lines record administrative convenience
Boundaries following parallels and meridians typically date from periods when territory was allocated between distant powers with limited local knowledge.
Such lines were quick to specify and could be surveyed later, which suited agreements made before anyone had mapped the interior.
They cut across drainage basins, grazing routes and language groups, and the consequences fall on populations who had no part in drawing them.
Independence generally froze existing lines
Newly independent states largely retained the administrative boundaries of the preceding arrangement rather than redrawing them along other principles.
The reason was practical. Reopening every boundary at once would have produced overlapping claims across whole regions with no mechanism to resolve them.
The trade-off was durable disagreement in specific places, accepted in exchange for stability across the far larger number of boundaries nobody contested.
Demarcation is a separate step from delimitation
Agreeing a boundary in words is delimitation; marking it on the ground with surveyed pillars is demarcation, and the two can be separated by decades.
Undemarcated boundaries invite incidents, because farmers, patrols and local officials act on different understandings of where the line runs.
Joint demarcation commissions exist precisely to close that gap, and their work is slow because every pillar location must be agreed by both parties.