The use of drones in modern warfare has become increasingly prevalent in recent years. Drone swarms can be difficult to track and defend against using anti-air defense. 

One potential solution to this problem is the use of mobile ground-based laser system to combat drones. 

• The biggest disadvantage is the cost of implementing such a system. Drones are cheap. Lasers are expensive. 
• Another disadvantage is the potential for bad weather to disrupt its effectiveness. In poor weather conditions such as heavy rain or fog, the laser beam may be diffracted or scattered, reducing its accuracy and effectiveness. 
• The phenomenon of thermal blooming means that laser weapons bleed off energy when penetrating the atmosphere, limiting their effective range, even in good weather conditions. 
• Laser weapons are limited by their need for a direct line-of-sight.
• Laser weapons are limited by the diffraction limit (see below). 

These are severe limitations that plausibly will prevent laser weapons from being deployed on the battlefield.

A potential way to get around the last two limitations is by deploying a (mobile) laser system together with an interlinked network of "mirror drones" or "selenic drones". These 'selenic' drones would carry mirrors that are able to redirect the initial laser weapon, allowing for much longer range and the ability to fire beyond direct-line-of-sight. This is similar to how moonlight is a reflection of the fact that the Moon acts as a giant mirror for the sun. Let us therefore call this system a helioselenic telescope or helioselenic hive. 

The use of mirror-equipped selenic drones allows for a significantly increased range for laser weapons.  By using mirrors to redirect the laser beam, it is possible to effectively fire around corners and over obstacles, greatly increasing the range of these weapons.

Another advantage is the ability to rapidly reposition the mirror drones as needed. In the event that an enemy drone swarm is moving or changing position, the mirror drones can be quickly repositioned to maintain the laser's line-of-sight, allowing for continuous engagement.

In SPACE!

Even if helioselenic hives are not cost-effective against drone swarms in the near future similar systems may still appear in the Far Future.

At the ranges and velocities that future space warfare is likely fought guns are mostly useless as an offensive weapon.  Missiles dominate. A missile travelling at many tens or hundreds of km/s would carry so much kinetic energy chemical warheads would be largely superfluous. A single hit would probably mission kill a spaceship. 

 The fundamental logic of space warfare is that missiles are cheap and plentiful and there is no stealth in space. For the cost of a single large spaceship one could plausibly buy many hundreds of missiles. Laser weapon systems could be viable only it they will be able to serve as an effective missile defense system.

In Space warfare the lack of atmosphere mean that there is no thermal blooming which greatly increases the effective power and range of laser weapons. But laser weapon range is not unlimited. In fact, space combat is probably fought at distances of many hundreds of thousands of kilometers or more. Laser weapon range is quite small compared to these ranges. The main problem is the diffraction limit. 

From http://www.rocketpunk-manifesto.com/2009/08/space-warfare-v-laser-weapons.html :

Over short distances, relative to the length of the laser itself, laser beams cheerfully ignore the inverse square law that governs ordinary light sources. But thanks to diffraction, over long distance they are effectively subject to it. The formula for the spread of a laser beam (via Atomic Rockets, of course) is a close cousin to the formula for telescope resolution:

$RT=0.61\ast D\ast L/RL$

where:

 RT = beam radius at target (m)

 D = distance from laser emitter to target (m)

 L = wavelength of laser beam (m) 

RL = radius of laser lens or reflector (m)"

(...)Let's get a bit more SFnal about it and specify a 100 nanometer UV laser firing through a 10-meter telescope, with beam power of 1 gigawatt and range of 5000 km. Our spot size is unchanged, but each square centimeter is now getting hit with about 8.5 MW, and you'll burn through a meter of graphite in a second. This is some serious zapping. Or you can achieve a 1 mm/second burn rate at 160,000 km, more than half a light second.

Of course there is a tech challenge or two: Operating a laser cannon is loosely comparable to mounting a jet engine at the eyepiece of an observatory grade telescope. You will produce waste heat greater than beam power, probably several times beam power. But all this merely makes it difficult, not impossible. Real lasers presumably won't be as good as ideal ones, but there's no inherent reason why they couldn't come reasonably close to diffraction-limited performance.

Using a helioselenic telescope of two dozen selenic drones a warship carrying a laser weapon could plausibly increase its effective range 20x or more. A nuclear-powered aircraft carrier sized space-ship (100 gigatons or plausibly even larger ~1 million tons) carrying a 1 GW laser and supported by a swarm of hundreds of selenic drones could plausibly withstand missile swarms of hundred or thousands. 

When defending against missile swarms a selenic drone-web has an additional advantage over extending the effective range of laser weapons. A selenic mirror -drone would be able to redirect the laser beam from many angles which makes it significantly harder to harden a missile against laser weapons. At a minimum one would need to armor the missile all around to effectively harden missile against a dense web of selenic mirror drones. For some components of space ships and missiles it may be quite difficult or expensive to effectively harden them:

But there is another and even more curious implication of laser combat. So far I've been talking about beams concentrated down to blowtorch intensity, kilowatts or metawatts per square centimeter, able to burn right through refractory materials by heating the surface to thousands of degrees K. But what about mere scorch intensity? Say, the 50 watts/cm2 that causes primary thermal burns to humans and sets paper on fire. This won't burn through armor, but it will likely burn out delicate components such as sensor elements, or at any rate saturate and 'dazzle' them.

Thus laser weapons can blind the enemy, temporarily or permanently, at much greater range than they can do serious physical damage to structures. Our first modest laser has a scorch range of 1300 km; the more SFnal one a scorch range of 2 million km … and the jumbo X-ray laser has a scorch range of 2 billion km, about 14 AU. Spot size (and targeting resolution) is wider by the same proportion, dozens of meters. More rugged sensors are the solution, but it seems likely that weapon lasers can dazzle or blind targets at several times the range at which they can burn through armor.

Combining laser blinding with electronic jamming plausibly could render a missile completely blind and therereby ineffective. Combining these measures with kinetic anti-missile defense it is plausible that very effective missile defense systems are possible in a SPACE! environment.