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In 1997, the US tested "illuminating a satellite" with a laser:

https://www.flightglobal.com/anti-satellite-laser-test-is-successful/17857.article

...achieving up to 10s continuous illumination against a US spy satellite at low power. That was 30 years ago, and it was already a given that firing that same laser at full power would have melted the satellite, even though the beam was 2m wide.

Modern lasers can get tighter and more powerful beams, the main issues are power usage and overheating. Something like the lasers at the NIF, can achieve fusion... or could be targeted somewhere else, if properly cooled down.

In 2023, China is claimed to have achieved enhanced cooling for continuous laser firing:

https://asiatimes.com/2023/08/china-claims-laser-weapon-gain-on-us-space-dominance/

...the title is obvious BS, in light of the 1997 US tests, but it makes one think the US likely has had something very similar for quite some time already.

Smaller systems have a smaller range, because... well, they're smaller. Fitting a laser system onto a ship, has some space, weight, and power limitations. Iron Dome ones are fitted to ground vehicles... and yet:

By 2023, energy levels could reach 100 kW or more and the system could focus a beam to the diameter of a coin at a distance of 10 km

https://en.m.wikipedia.org/wiki/Iron_Beam

That's gonna hurt.

The thing about ICBMs, is that they may enter the atmosphere at hypersonic speeds... but there lies the key: "enter the atmosphere"

https://en.m.wikipedia.org/wiki/Ballistic_missile

The trajectory of an ICBM takes it out of the atmosphere, up to 4500Km, where it is well visible from the ground. Even moving at up to 8Km/s on reentry, that gives several minutes for a laser to hit them, and up to a minute during atmospheric descent itself.

Missiles like the Iskander, reach an altitude of 50Km at 2Km/s... which again gives about a minute since detection, for a laser to burn one to a crisp. Not to mention conventional missile interceptors, launched from close to its target, can easily hit one too (interceptor speed is irrelevant, when the attacker is basically aiming at them).

Now, you're right about atmospheric conditions, and the beam can only be collimated so much (although some amazing progress is being made on that front too), and the longer the beam has to travel through atmosphere, the more it gets scattered.

Still, at current power levels, having several of these laser systems spread over a country would make it essentially immune to ICBMs, very hard to hit with semi-ballistic missiles, and leaves low flying drones and missiles as a best option to ever reach a specific target... which means flying in thicker layers of atmosphere, increasing propellant consumption and reducing their range.

Paired with conventional countermeasures against rockets and mortars, it makes for a pretty decent defense system. The US has also shown interest in acquiring some Iron Dome to complement its Patriot systems, which would seem like a winning combo.

Overall, right now it looks like in a total nuclear WW3 scenario, only missiles launched from subs would stand any chance at reaching any targets, and even that would remain to be seen. That's definitely a far cry from a 1960s MAD scenario... which makes it somewhat more scary, because someone might think the outcome could be actually acceptable.