This feels like a cop-out answer to me, especially on a world-building site, but I'm going to say that there's no way to answer this. Let me explain:

First of all, either the technology is discovered/learned by both sides at effectively the same time (on military timescales), or it wasn't.

If it wasn't, then whichever side got it working first would use it to win the war... unless that side didn't have sufficient forces to delay the invaders while simultaneously transporting an invasion force to inside the enemy capital's defenses. That lack could be due to manpower, geography, or a specific limited resource. Anything where taking troops off of the front would cause the front to collapse. However, assassination suddenly becomes a viable option even for a lone operative transported into place.

If they do have sufficient resources to open the new front via teleportation, they can make either targeted strikes on the enemy government/leaders, or send in a general occupying force to force a surrender.

If it was close enough to be simultaneous, then there's the next decision point: Either both sides know the other has (or will soon have) this technology, or they don't. If they don't know, than expect to see the "if it wasn't" tactics carried out by both sides, and whoever is first to do it will probably be the victor. If they're both aware of each other, it gets very interesting, and you can start drawing parallels to the Cold War.

Basically, each side has the ability to launch a devastating attack on the other, even if it means they are also destroyed. Depending on the leaders involved and the reason for the war, both sides may choose this option (or one may choose it, triggering the other to choose it), which will then, depending on whether the citizenry martyrs the dead leaders or goes "whew, we're done with war" either end the war or lead to renewed hostilities in revenge.

If neither side chooses to be the first to act, they'll start using the threat of being able to do so to negotiate. Since each side is equally vulnerable, the most likely outcome is a ceasefire along the current border, while each side tries to come up with some sort of defense. This is very much what happened with nuclear arsenals during the Cold War - neither side wanted to trigger open hostilities, but each kept trying to maneuver for advantage in other ways.

In short, there's just too many variables to give a simple "increase" or "decrease" answer. It depends on how simultaneous it is, the relative strengths of the ground forces, the willingness of the leaders to order assassinations of their opposites, the will of the population to keep fighting even after a leader is assassinated, and just how quickly each side can act.

This feels like a cop-out answer to me, especially on a world-building site, but I'm going to say that there's no way to answer this. Let me explain:

First of all, either the technology is discovered/learned by both sides at effectively the same time (on military timescales), or it wasn't.

If it wasn't, then whichever side got it working first would use it to win the war... unless that side didn't have sufficient forces to delay the invaders while simultaneously transporting an invasion force to inside the enemy capital's defenses. That lack could be due to manpower, geography, or a specific limited resource. Anything where taking troops off of the front would cause the front to collapse. However, assassination suddenly becomes a viable option even for a lone operative transported into place.

If they do have sufficient resources to open the new front via teleportation, they can make either targeted strikes on the enemy government/leaders, or send in a general occupying force to force a surrender.

If it was close enough to be simultaneous, then there's the next decision point: Either both sides know the other has (or will soon have) this technology, or they don't. If they don't know, then expect to see the "if it wasn't" tactics carried out by both sides, and whoever is first, will probably be the victor. If they're both aware of each other, it gets very interesting, and you can start drawing parallels to the Cold War.

Basically, each side has the ability to launch a devastating attack on the other, even if it means they are also destroyed. Depending on the leaders involved and the reason for the war, both sides may choose this option (or one may choose it, triggering the other to choose it), which will then, depending on whether the citizenry martyrs the dead leaders or goes "whew, we're done with war" either end the war or lead to renewed hostilities in revenge.

If neither side chooses to be the first to act, they'll start using the threat of being able to do so to negotiate. Since each side is equally vulnerable, the most likely outcome is a ceasefire along the current border, while each side tries to come up with some sort of defense. This is very much what happened with nuclear arsenals during the Cold War - neither side wanted to trigger open hostilities, but each kept trying to maneuver for advantage in other ways.

In short, there's just too many variables to give a simple "increase" or "decrease" answer. It depends on how simultaneous it is, the relative strengths of the ground forces, the willingness of the leaders to order assassinations of their opposites, the will of the population to keep fighting even after a leader is assassinated, and just how quickly each side can act.