This impressive 7,000 word essay written by user Memphet’ran on the appropriately named Spacebattles.com forum attempts to answer that question.
There are only a handful of engines that allow a combination of high thrust and low mass ratio. The most promising are Orion nuclear pulse propulsion and the nuclear salt water rocket. Some nuclear thermal designs also have thrust high enough to possibly be useful, although only for a small ship. The user “RJP” on Spacebattles also suggested something called a fission fragment drive which works by throwing high-velocity fuel fragments out the back of the ship, but other sites I’ve researched suggest it would be a low-thrust high-ISP [in-space propulsion] system more suitable to an explorer than a warship. Orion works by the (seemingly insane, but actually quite effective) method of throwing nuclear bombs behind the spacecraft and having it ride the blasts. The hot gasses from the detonations hit a heavy pusher plate at the back of the ship and drive it forward. NSWR [nuclear salt water rocket] is similar, but it instead uses a solution of fissionables in salt water that spontaneously explodes as it leaves the rocket nozzle. Both systems cleverly shift the propulsive reaction outside the spacecraft, eliminating the need to deal with most of the heat it produces and allowing it to be made much more energetic.
For the same reason that we have Space Shuttle launch delays, we’ll be able to tell exactly what trajectories our enemies could take between planets: the launch window. At any given point in time, there are only so many routes from here to Mars that will leave our imperialist forces enough fuel and energy to put down the colonists’ revolt.