For example, very large lasers have been proposed for propelling light
sails. A laser several tens of thousands of kilometers across could
focus intense light on a target at interstellar distances (assuming you
know where the target will be in a few years).
Starships themselves can be adapted as weapons. A 1000 ton starship
travelling at 1/2 the speed of light has the energy of several teratons
of high explosive. An unmanned "bus" with a large tracking telescope
could carry hundreds of thousands of kilogram-sized submunitions to
a target star system. Several light weeks out it would aquire targets
in the star system (perhaps optically, or by detecting radio emission)
and release the submunitions, which would be tracked with high precision
and guided onto trajectories intersecting likely targets. Before
impact each submunition would disperse into a cloud of dust and gas.
Each 1 milligram dust particle would hit with the force of a several ton
bonb; the gas molecules would simulate a pulse from a neutral particle beam
accelerator. Such a weapon might be easier to build than a starship,
since it needn't slow down upon arrival.
While it is certainly possible to make intersteller war, the
question is 'why bother?'
What possible motive could be concieved for making a war?
Conquest? Kinda hard to adminisrate a planet when it takes
several years to communicate with it. Eventualy, it would revolt
successfully.
Idiological? It is hard enough to get one country to agree on something,
let alone a planet.
Resources? Seems kinda wasteful to attack one solar system when there
are others at about the same distance (including your own)
-cory