I can't recall the name of a term, phrase, or name of a general used to bring all of your assets to bear against one of many foes/problems at a time, instead of all of them at once. The presumption is that this technique is used in a defensive way.
As I recall, it was a military maneuver (could be strategic or tactical) that referred to how to deal with many enemies that were attacking at once. Instead of attacking them all at once and wear them all down over time, one should attack each one in turn until it dies, and then move on to the next one, ultimately whittling down their offensive capabilities.
I want to say the maneuver came from Russian forces in a spectacular battle, and was thus epitomized.
I'm sure this concept can be used in other disciplines, for example, war games, strategy games, action games, Dungeons and Dragons, etc.
Force Concentration and divide and conquer (aka defeat in detail) are both close in concept, but I didn't get the "aha" moment from reading them, so I'd like to add examples to make sure I'm describing it correctly. Divide and conquer usually applies to offense, and I think the technique I am asking about is more for defense.
Tactical example using Dungeons and Dragons
The party is facing a group of monsters. They have two choices. Each party member can attack a different foe and eventually the party may win. Or all the party members can attack the same foe, and they kill each monster in turn.
In this example, there is not enough room space to divide the monsters into separate groups in any meaningful way, so I don't think divide and conquer is the proper term.
The monsters will continue to attack every round, but a key point of the phenomenon I'm asking about is that this technique removes the number of attackers faster, reducing the amount of damage taken per round compared to the alternative.
Strategic example from Legend of Galactic Heroes
I think the first battle in the first episode of Legend of Galactic Heroes is a strategic example of the technique I am asking about. This page has a more complete description of the battle if you haven't watched it. Quote below.
In this case, you could argue that the enemy divided themselves to let another conquer them, but don't let the details of the situation detract from the act of taking out one enemy at a time.
On 1 January 796 UC, the Alliance 2nd Fleet, 4th Fleet and 6th Fleet entered the Astarte Starzone. Their plan was to surround from three sides and annihilate the smaller Imperial fleet. This was a re-use of the Alliance strategy at the Battle of Dagon in 640 UC, which had been a decisive Alliance victory.
Reinhard immediately saw through the rather simple strategy, and ordered his fleet to advance towards the 4th Fleet, under the command of Vice Admiral Pastoll, before the trap could be executed. Attacking the single fleet whilst the other two were out of range, Reinhard would hold the numerical advantage. At this point, Commodore Yang Wen-li, a strategic advisor to Vice Admiral Paetta of the 2nd Fleet, recommended the 2nd Fleet immediately join up with the 6th Fleet, and then move to attack the Imperial forces. Paetta refused, opting instead to move in to support the besieged 4th Fleet. Lohengramm's break-through attempt
By the time the 2nd Fleet arrived, however, the 4th Fleet had been all but obliterated, and Reinhard's forces were already engaged with the 6th Fleet. After defeating the 6th Fleet, Reinhard turned to the 2nd. Admiral Paetta was severely injured early in the battle, and Commodore Yang assumed command of the remaining forces. (LOGH: 'In the Eternal Night')
Yang ordered the 2nd Fleet to attack the Imperial forces at their own discretion, and after a bit of steady fighting, Reinhard ordered his fleet to take a spindle formation to break through the enemy's lines. Yang took advantage of this: whilst the Imperial fleet broke through, the Alliance ships pretended to be routed, splitting into two groups, which then assaulted the Imperial fleet from behind. Reinhard was forced to turn to attack, resulting in a massive ring formation between the two fleets — like two snakes devouring each other from their tails. Unwilling to waste time and lives in a pointless battle of attrition, Reinhard retreated from the Astarte Starzone, allowing Yang to do the same.
2,450,000 Imperial soldiers survived the Battle of Astarte. The Alliance survivors numbered 4,060,000. However, the number of Imperial casualties came to only 150,000, whilst the Alliance had ten times as many, at 1,500,000 casualties. Though the Battle of Astarte was an enormous loss for the Alliance, Yang's successful retreat allowed the Alliance leadership to portray the battle as a 'victory', and to name Yang, already the 'Hero of El Facil', the 'Hero of Astarte'. (LOGH: 'The Battle of Astarte'; Overture to a New War)
The recently remade version of the anime explains this in more detail during the battle, and you can watch it on Crunchyroll, though I can't grab the link for technical difficulties.