The problem with laser weaponry is 3 things:
1) laser energy is extremely inefficient to produce. Less than 1% of the energy input leaves as beam energy. Nearly all of it leaves as heat. In our world where the second law holds the whole universe by the balls, this is a serious issue. Laser systems need to be large and bulky just to dissipate the produced heat.
2) production of the necessary energy to power a high energy device is nontrivial. A 500MW beam weapon requires gigawatts of electricity just to make the beam, and more energy to dissipate the heat.
3) in atmospheric conditions, there is a maximum beam energy you can release before you develop "bloom". "Bloom" is essentially the result of ionizing the air the beam is traveling over, and inciting it to become plasma. When that happens, it becomes opaque, and scatters the beam. This means your laser weapon suddenly falls off on effective power faster than a man with ED. To resolve that issue, laser weapon systems use "pulse" lasers, which pulse the photon energy in high intensity spikes, with a refraction period for the air to cool off again before the next spike. However, this comes at a cost, as the laser takes time to reach full intensity. This is called the avalanche period. Not all mediums are good choices for laser weapons as a consequence, and the effective beam conversion drops even lower than it already is.
This is why laser weaponry systems are not considered viable for warfare, except for very specific, specialist applications. No phasers on roast. Sorry.
A klystron, on the other hand, is something radically different.
It is more like what you get when you rub a wet finger on a fine crystal goblet. A light energy input resonates the goblet, and it rings loudly and profoundly, for as long as you keep rubbing the rim.
A klystron is essentally a long, metal tube that is hollow, and evacuated.
Inside this tube, an electron beam gets fired. Such as is found in old glass TV sets.
At the near end of the tube, a microwave input antenna broadcasts a reference microwave signal. This signal pushes on the electron beam ever so slightly, and causes it to wobble. The wobble of that beam resonates the tube perfectly in harmony with the reference signal, and the whole tube starts to emit microwaves of the same frequency. Those waves further perturb the beam inside, and eventually you have a veritable microwave "scream" coming out the far end. It is very efficient, and produces sufficient microwave emissions at the far end of a long drift tube to kill birds and cook hotdogs, or worse.
Kystrons are the heart inside things like the US navy's ship radar systems, and coastal radar monitoring networks. They are more than 60% efficient at converting electrical energy into highly focused microwave beams.
Given the very short distances involved, a tower mounted, high energy klystron would make very short work of an invading army. Very literally, it would be like popping them in the microwave on high. They would die in minutes, and feel the effects in seconds.
Metal suits of armor would weld together at the joints, and bodies would cook from the radio excitations, exactly as if they were placed inside a microwave oven.
The kystron is easier to build, easier to maintain, more efficient, invisible, and devistatingly deadly if built powerfully enough. It is also quickly on/offable with just a simple switch.
It also makes a nice radar detection signal source.