Always heard none, but am thinking that, for example, a 100k army from 200k is still nothing to fuck with.

Just random curiosity.

It's not just a matter of size/numbers. It depends on many factors.

Type of conflict, size of conflict, terrain, adversary, leadership, morale, level of training, logistical support, technology, offensive vs. defensive action, etc.

Smaller forces with other advantages routinely hold their own against numerically superior enemies. Look at the Spartans against the Persians at Thermopylae. Or the English against the French at Agincourt. Or the British against the Zulu at Rorke's Drift. Or the Finns against the Soviets in the Winter War. Or the Green Berets and Northern Alliance against the Taliban in Afghanistan.

Just one RAMBO

In addition to the factors mentioned by RogueJSK, there have been many units throughout history that were still considered combat effective after being reduced to 50% or less of their original strengths.* How effective is the question, but they were still fighting as cohesive units. Examples included units of the German army in WW II. But suffering heavy casualties can do more than reduce the number of bodies in the organization, and those effects can affect combat effectiveness. When they’ve had enough, for example seeing every other man become a casualty, they may stop fighting as well, if not just quit (e.g., the mutinies in WW I).

* Some units have suffered more than 100% casualties throughout a war without being reduced to less than 50% of original strength due to receiving continuing supplies of replacements.

The term 'combat ineffective' does not mean 'useless.' It simply means it cannot be expected to perform its assigned tasks adequately. For example, a brigade at 50% strength is still more powerful than a platoon. But, it cannot be expected to do a brigade's work. Also, that many casualties disrupts the chains of command (if all a unit's officers or NCOs are killed, that unit will suffer badly even if it's nowhere near 50% gone).

Also, it's a guideline more than a rule. As stated earlier, many units have fought on (some successfully) after suffering 50% or greater losses.

It also depends on the enemy. That 50% brigade should still be able to obliterate the 100% platoon (but not a 100% brigade).

Also also, nukes. If China or Russia lost 50% of their nukes, they would still remain a threat with the remaining nukes, since even a single nuke is a threat that cannot be ignored.

As I had thought then.
Thanks

Any Israeli unit.

The Israeli's have a secret weapon.

It is called; "Ein breira".
אין ברירה

No alternative.
or
No (other) choice.

a unit sustaining that kind of loss should be reorganized.

and the the missions tasked to that unit would need to take that into account until it is brought back up to strength

as mentioned above -- a key missing component would be LEADERSHIP: a platoon of 15 men led by an E5, no squad leaders and the rest junior enlisted cant accomplish anything useful. and on and on up the chain...

Take the same 15-17 men and give them an LT, a strong E6 and a couple solid E5s and you would be MUCH better off even though the manpower is essentially the same. from what I have read -- units in Vietnam often functioned with this type of manpower

Hitler was famous toward the end for commanding fictional units which were no longer fit for combat either because of attrition or mass surrender.

Also another reason when you read mil history they almost always talk about maintaining adequate reserve forces to buttress locations where units take heavy casualties.

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The numbers vary depending on the source (12,000-15,000), but the Confederates lost about 50% during Pickett's Charge (many were just captured). Had the rest kept fighting instead of retreating, they would have become casualties too. However, Lee kept 5 brigades in reserve. Had he put them in, he would have captured the Federal position. All according to: Civil War Mathematics

Using a Battalion as an example 50% casualties would be crushing.