"Most Difficult to Traverse?" Topic

Which type of terrain is most challenging for an army on the march: a mountain chain, a desert, or a large river?

Oof. Hard question. All things being equal, probably mountain chain. Mountain chains may incorporate the desert or the large river or both, with the added glories of winds, falling rocks/landslides, avalanches, fantastical temperature differential, limited or nonexistent water sources….

Napoleon ranked them in ascending order of difficulty as rivers, mountains and finally deserts. For modern armies the considerations are a bit different, but for horse powered ones, I'll go with Napoleons assessment.

How far do you have to go? Do you have bridging materials? Are there enemy forces in the area?
In general, I can go with Napoleon.

If the passes through the mountains are already known then I would agree with large rivers.

I think that a swamp should be added as a type since it has the difficulty of a river plus one can be more extensive & treacherous.

In general swamps can be avoided – while they may be large and difficult, an army can choose an alternate path. If you are not facing an immediate opponent, then the desert would be most challenging.
Opposed river crossings are great ways to lose a lot of troops and mountain passes allow a numerically smaller opponent to concentrate forces.
I would give a slight nod to mountains as they can become impassable during the winter – which limits the attacker's options in time as well as funneling into space.

Seas and oceans gave severe problems.

Even Napoleon had trouble with them.

I'm not sure this is a meaningful question. Which river? Crossing the Rubicon or the Amazon isn't quite the same thing. Similarly for deserts and mountain ranges.

One consideration is how can the logistics tail keep the army supplied as or after they cross the desert, mountains or river.

Also see the opportunity of piercing a cordon defence of a river or mountain range to isolate portions of the defenders.

I would put the desert as the toughest challenge.

As for my locale, I would say the hardest is Interstate 85 from 17:30-19:00 daily😏

As a reenactor the toughest terrain I ever faced, oddly enough, was a freshly harvested cornfield. The bits of snapped off cornstalk were as slippery as ice. Marching my battalion through them cut our speed by about 75%.