Is there a common term for something that drastically decreases a military fighting force’s effectiveness, like the opposite of what would be called a “force multiplier?”
Yes.
It’s called “a bad idea”. 
Seriously, I don’t know of any special term that would be required.
Don’t know of a term as such, but “terrain”, is a often used as a one word reason as to why military forces are suddenly a lot less effective, though obviously it on its own does not mean that.
Adversity in any form is clearly… adverse.
I thought the OP was referring to something that could be acquired, as opposed to encountering it. Maybe I’m wrong.
“Vulnerabilities”, maybe?
Well, obviously the antonym of “force multiplier” would be “force divider” or something very similar, and in an appropriate context even someone who had never heard the term would probably understand it, so long as they were already familiar with the term “force multiplier”.
If “force divider” doesn’t occur in nature, so to speak, or is vanishingly rare, that would suggest that the concept it describes is not spoken of enough to require a term.
Since virtually anything positive can be cited as an example of a force or combat multiplier from ensuring troops are paid on time or are able to get mail, let alone anything involving actual firepower or military supply logistics, virtually anything negative could also be the opposite of a combat/force multiplier. As such, I don’t think there is any one word or term that is universally used to mean the opposite of a combat/force multiplier, but I have heard the phrase combat/force divider or force/combat detractor.
Yep, in Center of Gravity analysis, a critical vulnerability is pretty much that. So if an enemy did CoG analysis on us (US), it might point to things that would be particular force dividers or degraders. (not all CV fit though)
How about a “force nullifier”?
“Degrader”, in my work environment.
Don’t know if it’s used that way - but logically, a negative number can also serve as Multiplier, so … “negative force Multiplier”?
Good temperature /Weather condition: troops gain by factor of 1
Too cold/ too hot/ rain: troops loose effectiveness by factor of -2
Morale high: factor of x2
Morale low: factor of x-5 to -10
I wonder if Video games are programmed like this?
force reducer, perhaps?
Constraint.
Vulnerability?
This sounds good.
Nullifier would imply that the force level was brought to zero (or null). I believe the OP is looking for a term describing a reduction in force.
“Force multiplier below unity”
The proper term is “cadorna.”
Ding!
In some contexts - for example, an army purchases a piece of equipment that causes unexpected problems - it could be “liability” or “burden”.