. More seriously though, consider the following situation. ...actually, do that at your discretion. It got a bit out of hand, but I was trying to maintain a level of fun as opposed to getting out my physics penis and having a great big penis waving competition. Sci-fi should be fun. I hope it's still an entertaining read, but it might bore some viewers. The only really relevant point is that chemical fueled missiles for light second range combat is ridiculous; evidence: the moon missions. Think about it, or search the relevant information below.
Let us say I have placed my faith in an unstoppable death ray of supreme range; a large orbital fortress with a relatively high frequency laser. Being more conservative and shamelessly cribbing Luke Campbell's example from the atomic rocket weapons page, at a light minute on a hit it'll take a matter of milliseconds to start vaporizing hull metal. At a light second, its more like microseconds.
An invading fleet from Mars decides it really doesn't like the cut of my jib and launches a two-pronged attack. The sneaky martian bastards have deduced that the huge radiator array on the orbital fortress isn't just for show and is in fact for radiating, and aim a mass driver launching loads of Playboys and tennis balls and banana peels at my radiators, while fueling a squadron of missile boats to come into my neighborhood and rumble.
I am on Earth for the purposes of this example. Chilling, if you will. If I'm lucky the sensors monitoring the martians for such shenanigans note the discharge of the mass drivers, everyone in the solar system notes the missile boats taking off. The martians decide they're done fucking around and accelerate up to about 100 km/sec. I'm not sure how comfortable I am with the solar system trappings of this metaphor because relative orbital positions of the involved planets make things complicated but what the hell let's roll with it.
So the martians get up to speed and decide, ah, fuck it they're done burning fuel and using up reaction mass, they'll coast in on the tail of the mass driver payloads. The commander of the orbital fortress (nicknamed "the Death Star" by its geeky crew) orders earth system freighters to start shipping him all the water and other coolants they can get their hands on, and start firing. What the hell, maybe you'll get lucky.
The captains of the martian spaceships note the death star has started firing and are a little panicked, as my lazy math has them about a week out. They dodge dutifully and are duly missed, but after about a day of serious maneuvering the crew is sick of pulling 1.5 gees and the fuel tanks are showing some strain. Eventually one ship gets hit - pure luck - and suffers minor damage. Electronics shielding prevents major damage to ship systems, but there's some minor irritation in the sensors.
So the martians sluggishly duck and weave to conserve reaction mass the whole way there, occasionally getting tickled by a beam. One or two improperly shielded missiles get disabled by the death star, but no biggie. Fast forward to day 6; the humans haven't really gotten lucky again and stopped firing to dissipate heat. Space suited hunams have placed bags of gravel in the way of the metallic (and radar-detectable) mass driver payloads and the martians still haven't gotten into the light second ranges they need to start considering deploying their missiles. These hunams have been making them sweat bullets for most of a week, no way they're launching their birds and turning tail. The Death Star cools it for a day, bleeding off excess heat, and then its time for the final showdown.
The martians reach a light second of range and then the shit hits the motherfucking fan. The death star starts firing, and this time it's not a harmless spotlight, oh no. It's depositing a couple of terawatts per cubic centimeter in iron-like spaceship armor; who knows what the martians would really armor their ships with but whatever it is it hurts. Bad. Even at 100 km/sec, in a microsecond the martian cruisers have only moved ten centimeters, and using target tracking equipment that was cutting edge in the era of the Phalanx CIWS the death star's turret was tracking the ship's movement. The first hit puts a hole straight through the armor, crew quarters, munitions bay, maneuvering propellant tank, and bridge of the lead martian cruiser. Chaos reigns, the ship just loses its shit all at once. There's a gash-shaped breach that came out of nowhere, and the second in command can barely radio to the other vehicles to begin evasive maneuvers and deploy munitions before his console explodes in a shower of cheap visual effects.
Hundreds of kill vehicles are released from each martian cruiser and they start braking like madmen. They don't want to be near this thing for one god damn second longer because it'd take tons and tons of armor to deflect a glancing blow, and a direct hit transfixes a ship.
The martian missiles wake up, give each other electronic bro-poounds, and coast at 100 km/sec towards the Death Star. There's a thousand of them; some armed with live warheads, some armed with bubbly personality to lure away real point defense, some armed with submunitions to engage point defense. But they notice something funny; near the death star there's a lot of small objects.
Now here's an informative aside: it took the upper stage of a saturn V rocket to put three people about a light second away. It took the better part of 3 days each way, and 95% of the course was ballistic (ie eminently shootable). I grant you in this scenario the missiles don't need to boost up to speed, but they do need to make sure they hit the target. And dodging is energy intensive. A laser just has to exert enough force to make a hole through the target. An engine trying to dodge has to physically move the entire target out the way.
Anyway, where were we? The martian swarm of death notices that the Death Star has deployed countermeasures and stopped firing on the martian cruisers. It has also reduced shot power and has begun firing on the incoming missiles. It doesn't need to put a hole through a warship, just scoop a few centimeters of armor off the engine of a martian missile. It won't detonate in a ball of fire, but it will also be unable to safely maneuver. Low power shots are fired, the very dangerous swarm of missiles begins using its fuel to dodge like crazy. And to their credit, the swarm of missiles succesfully dodges; it takes multiple shots for the death star to kill a single missile and the countermeasures are moving to block the new courses of the missiles.
So for 300,000 kilometers, the martian missiles are whittled down by ever more accurate beam fire; the Death Star's firing constantly and these aren't ICBMs. With ICBMs the defender has to decide in a matter of seconds which is a threat and which is a hot aluminum balloon. The DS has 50 minutes and has shot the countercountermeasure missiles, the missiles with lasing rods strapped to them, and who knows how many others. Out of 1000 initial missiles, perhaps 600 remain, and the DS deployed two thousand countermeasures. Countermeasures need fantastically less in the way of fuel, a warhead, guidance, or shielding; their job is to GET IN THE WAY. How the hunams financed the death star and all these decoys is an exercise best left to the reader. In the scramble of seconds as the martian missiles close the range, all but one are interdicted by the valiant little countermeasures.
The last missile had furiously expended propellant maneuvering to avoid its interceptors and coasts into the target on fumes.
The death star swats it 50 kilometers from impact with a last ditch low power beam that vaporizes just enough of the armored missile nose to deflect it from an impact. The missile begins slewing like crazy to reacquire, runs out of gas, sighs to itself, and detonates as it passes a few hundred meters past the station, ensuring a total victory.
Except this missile's arming mechanisms had been cooked entirely by that last laser shot. Leisurely the defenders of Earth flush coolant and pick off the martian cruisers as they try and leave, but cooler heads note how lucky they were not to have been roasted by that last bomb.
Earth briefly considers formally declaring war on Mars, and then realizes that with the large lens on their Death Star they could shoot at Mars's decidedly unmaneuverable orbital industry with impunity and carve "FUCK OFF" into their shipyards.
. Not exactly centuries in the future, eh?