[I appreciate and respect your opinions, and point of view. It's obvious you've spent a great deal more time considering this subject than I have.

I view this discussion only as an extremely interesting (And imaginatively entertaining) thought experiment. It's not my intention to debate*, but more to flesh out how the course of events might, or might not unfold, driven by the gargantuan scale of interstellar space.

*Besides,...something tells me, I'd be the one left in “smoking ruins” ;)

On the other hand, if you've had this discussion one (Or ten) too many times, and would rather watch paint dry than go there one more time, I understand completely, and no hard feelings.]

Very good point, and I tend to agree with you. Conventional wisdom in military doctrine, says, get there “firstest, with the mostest”. The best defense is an a__-kicking offense.

Conversely, letting the enemy within your perimeter, is generally accepted as a recipe for suicide.

Another comment on conducting battle, is an anonymous quote, which I will mangle to read;
“Armchair Generals speak of Strategy and Tactics,...professional Military Planners speak of Logistics.”

I'm not saying you belong to the former group, but I'll tell you I certainly do. Even so, the message hasn't been wasted.
The larger and more complex a campaign becomes, the more crucial logistics are in determining the course of conflict, including the specifics of Strategy and Tactics. I'm sure you've already thought of the obvious examples from WWII. (Operation Barbarosa, the Normandy Invasion, the Pacific War)

Clearly, just one manned (Peaceful) interstellar mission to our nearest stellar neighbor, using the technology outlined by the author of the article, would be a tall order indeed. There is a real possibility, that it might never be possible. (Though I'd like to believe otherwise)
Assembling a fleet of interstellar warships capable of maneuver warfare at relativistic velocities, and launching a military campaign against, and engaging a similar force, in a war zone ~260 trillion miles long,..... That's some big stuff.

A WWII style supply train of support ships (Stretching back to the Home System) seems pretty silly, so, it's likely all the logistical infrastructure will have to be incorporated into the fleet? If so, it gets significantly larger and more complex.

How do you exercise command and control over this fleet while moving at relativistic speeds?

How do you detect the enemy fleet while either your's or both are moving at relativistic speeds?
Maybe I'm wrong, but this seems like it could be a real show stopper.

If you do somehow detect them, what action do you take?

A. Begin applying reverse thrust to slow down and engage them, (How long would that take?) in which case you use a great deal of propellant, (Which has to be a limited commodity in any case) and if you guessed wrong they go streaking by.
Do you re-accelerate to near light speed, knowing you'll never intercept them in time?

B. Maneuver to take a shot at them? If you're moving close to the speed of light, on a certain heading, and you somehow detect another ship, (Or fleet of ships) moving close to the speed of light in the opposite direction, but off your heading say a degree or two, how do you alter course in time?
More importantly, what do you shoot at them under those circumstances? What could close that gap in time? How would it be guided?

I can see how a stationary fleet could sit in ambush, and (If they're looking in the right place at the right time) detect an opposing fleet moving at relativistic speeds towards them. Finding it wouldn't be easy, but some sort of ultra-sensitive EM sensor, like an infra-red telescope should be able to see the thermal signature of the propulsion systems exhaust plume. But again, actually engaging a target (Or many targets) moving at such velocities, and hitting it? (them) I'd like to hear a good explanation of how that's done,...if it can be done.

Conducting an effective battle at such tremendous distances, and at relativistic speeds, seems so problematic, as to be a very bad gamble.
It is force projection that extends beyond the ability to control the battlefield, in space and time.
It might be an interstellar Battle of Jutland, or it could be a catastrophic mis-allocation of forces, that leaves the Home System essentially, wide open to attack.

Littoral Strategy:
I'm not suggesting we wait till they get to LEO, but what about the outer most shores of the solar system, the Oort Cloud? Some estimates have it extending as far as 3 LY radius, and not just along the plane of the ecliptic, but more of a globular halo.
The enemy might want to replenish propellant and consumable volatiles. At that point, they would be as vulnerable as they're going to get. It could be a good place for ambush.

Concerning the relativistic kinetic weapon you suggested. It is a terrifying thought, but I have a lot of trouble imagining something composed of solid matter, moving very close to the speed of light, and surviving against the Solar wind, especially in the inner Solar System.