>effortlessly

A reactionless drive, if possible, would still require "effort" to produce thrust in the form of energy. It is not a "free energy" device. The only difference between a reactionless drive and a rocket is that a rocket must carry its own fuel with it, so that it can expel the fuel to generate thrust. The fuel adds additional mass to the whole rocket, so in a way, a rocket requires extra fuel to carry its fuel. Since a reactionless drive does not expel anything, a reactionless drive would not carry any fuel, and therefore could be more efficient by way of lower mass, but it would still need to carry an energy source of some type, such as a nuclear reactor, or a radioisotope thermoelectric generator.

Rockets are certainly capable of accelerating a mass to a significant fraction of the speed of light. However it is important to note that it becomes more difficult to accelerate an object as the object gets closer to the speed of light. You need more energy to add a constant amount of velocity as your current velocity increases. This applies regardless of what kind of method you are using to accelerate - doesn't matter if it's a rocket or a reactionless drive. The difference is only in efficiency - it is thought that a reactionless drive might be more efficient. However if a reactionless drive really is possible, it might provide a lot less thrust than you think.

So even if reactionless drives are developed, they might be more efficient than rockets, but far less timely. I think you'd be waiting quite a while for the reactionless vehicle to attain any sort of appreciable velocity - perhaps decades. As far as weapons go, humans have not met any aliens yet (to my knowledge) and our species currently only resides on planet Earth, so any weapon would be ballistic in nature, to strike targets on Earth's surface. A reactionless drive would be ill-suited for ballistic propulsion because it doesn't give enough thrust. If it was launched from the ground like an ICBM on a suborbital trajectory, it wouldn't be able to escape Earth's gravity, it would just sit on the launch pad. If it was placed into orbit using a rocket as a first stage, and then activated later, it would take a while to deorbit rather gently. The vehicle's velocity would slow down due to air resistance, and its kinetic energy would not be usable as a mass destructive weapon. This is why large explosions are used as weapons of mass destruction, and not kinetic energy alone. Kinetic energy is simply used for timely delivery of the explosive device from a great distance away.

Now, for interstellar warfare, a reactionless drive could be useful if you were extremely angry at another planet in another star system located many light-years away. The large distance between the aggressor and the victim allows the kinetic vehicle a lot of time to accumulate velocity. However the steering of such a kinetic weapon must be very precise in order to hit the target planet, so the vehicle must have the means to estimate its current location/speed/direction, compute the orbits of itself and its target, and apply the necessary course corrections in order to hit that target. It is possible that a reactionless drive might provide vectored thrust, but if it does not, then attitude adjustment thrusters would be needed (either reactionless or conventional.) Accurately steering a vehicle traveling near the speed of light does not seem like an easy task. If the target was missed, the vehicle would be traveling so quickly that it would escape the star system on a hyperbolic escape orbit.

Such a weapon would be quite the undertaking and would be a large energy expenditure for a civilization. It might be more worth it to focus that energy into colonization and defense instead.