Military term or maneuver that describes focusing all your efforts on one enemy at a time

Solution 1:

You may be thinking of divide and conquer.

This is where you intentionally try to make your enemy split their forces into smaller sections. This can be done in a variety of ways, from creating a diversion ("Hey, you three, check out that noise"), making only specific entry points valid attack vectors ("You three go left, you two right, and the rest of us are going straight"), drive a wedge through them (attacking one specific spot in their formation so they naturally scatter), or a whole host of other options.

Many of these options could lead to leading or driving the enemy into a trap or ambush, where you have overwhelming odds. This make it so you can more easily defeat small sections, while not necessarily being able to take on their full forces.

You may want to divide and conquer simply to avoid massive losses on your own side, if things are relatively equal.

Edit:
A better term could be targeting.

The process of selecting and prioritizing targets and matching the appropriate response to them, considering operational requirements and capabilities. See also joint targeting coordination board; target. (JP 3-0) (US DoD)

https://www.militaryfactory.com/dictionary/military-terms-defined.php?term_id=5331

And more formally, to possibly include multiple branches or military units: joint targeting coordination board.

A group formed by the joint force commander to accomplish broad targeting oversight functions that may include but are not limited to coordinating targeting information, providing targeting guidance and priorities, and refining the joint integrated prioritized target list. The board is normally comprised of representatives from the joint force staff, all components, and if required, component subordinate units. Also called JTCB. See also joint integrated prioritized target list; targeting. (JP 3-60) (US DoD)

https://www.militaryfactory.com/dictionary/military-terms-defined.php?term_id=2942

Targeting would specifically be to have multiple soldiers, squads, units, or other groups attack a single target, rather than each engage their own targets. This could mean multiple soldiers with AT-4's attacking a single tank, multiple Seal teams entering a building from different entrances looking for a single bad guy, or even a group of bombers attacking a city with the intent to destroy industry buildings.

This is normally something done before a mission is started, so it's often used offensively, but a commander being attacked can decide to target multiple units at a machine gun nest if it's doing more damage than other enemy around it.

Even when there's a plan in place, it often needs to be updated with the real-time info coming back from the "boots on the ground". Something like a missile emplacement being discovered will likely warrant a reconsideration of what order to target enemy sites. Targeting the anti-aircraft site would likely need to be prioritized to happen before air support could commence.

A joint targeting coordination board would be a way for a commander or multiple commanders to coordinate troop movements among multiple Companies or branches of the military. Troops on the ground might call for artillery or air support to target a bunker or other hardened facility. Knowing that planes are headed for a specific enemy, instead of a refueling point or finishing off a previous bombing run could be something to put on this board to prevent the planes from being redirected.

If you are in a turn based game, you might be targeting the "tank" to make sure that one character doesn't wipe out your whole team. Or you might target the medic to make sure they can't simply heal or resurrect the "tank" every time you hit them.

In a RTS, you might target the commander, knowing that if they die, the whole basecamp/enemy dies. In a team based RTS game, you might target the weakest member of the team so that the other teammates have to expend resources or leave their bases open to attack to prevent losing the base. In some RTS games, there's a mini-map and the concept of a "flare" to bring your teammates into a coordinated attack. This could be considered a JTCB, of sorts.

Solution 2:

Not a single word, but this IS the military tactic:

defeat in detail wikipedia

a military tactic of bringing a large portion of one's own force to bear on small enemy units in sequence, rather than engaging the bulk of the enemy force all at once.

As in:

Justus D. Doenecke - 2013 - ‎Preview - ‎More editions Attempting to save the British Isles, flanked as they are by enemy bases, ringed as they are by submarines, and open as they are to air attack, stands a chance of being the greatest "defeat in detail" in history.

and

Journal of the Military Service Institution of the United States Such procedure necessitated initial deployment by large semi-independent groups over a very wide front, and consequently increased the danger of defeat in detail should the enemy succeed in concentrating on one or more unsupported ...

and

One definition states: “Defeat in detail is a doctrinal military term that means to defeat an enemy by destroying small portions of its armies instead of engaging its entire strength” (Erickson, 2003). tititudorancesa.com

Solution 3:

In an article entitled “THE ‘CENTRAL POSITION’ ARTICLE” in THE EAST ASIA OBSERVER,

the strategy of the central position (aka “the strategic central position”) is discussed and described in a way that seems similar to what you are describing, although they attribute it, not to Russia, but to Frederick the Great of Prussia:

The strategy of the central position originates conceptually in the defensive tactics of Frederick II (“the Great”) of Prussia during the War of Austrian Succession and, later (and especially), during the Seven Years’ War, in which Frederick’s outnumbered, gradually depleted, and increasingly exhausted army maneuvered between two (and sometimes more) foes with smaller individual forces; Frederick’s army could maneuver thus and react more quickly than those of its opponents—and, therefore, keep those forces from coalescing and forming a more formidable single force—due to Fredrick’s army’s operating on what Antoine-Henri, baron Jomini called interior lines. Because the distance between two or more points in a smaller or interior geometric shape are closer together than two or more points in a larger or exterior geometric shape, it takes less time—depending on organization and speed, of course—to trace (or, for an army, to march) along interior lines than exterior lines. Hence, due partly to geometric truths, Frederick’s army had a considerable advantage in speed and distance regardless of the other disadvantages with which it had to cope.

(see also, Wikipedia’s entry for this strategy)

Solution 4:

Although it isn't one word, this sounds to me like the use of interior lines:-

Interior lines is a strategy of warfare that is based on the concept that lines of movement, communication, and supply within an area are shorter than those on the outside. As the area held by a defensive force shrinks, these advantages increase. Using the strategy of interior lines, a surrounded force can more easily supply, communicate, and move its forces around, and can mount a series of surprise attacks on the forces encircling it. [Sensagent]

The source has some examples that you could look up to see if it is what you are looking for.

Skillful use of interior lines exposes the enemy to the risk of defeat in detail, as also mentioned.