Yeah something like as your spead increases so does your mass. But that wouldn't matter in space, now would it? It's all about counteracting that inertia so you can stop.
Ever read Ender's Game?
Earth was sending an armada of ships to numerous planets all occupied by a race of aliens.
But because it takes so long, Earth time, to wait for those suckers to get there, we keep increasing our technology and sending out more ships.
But the new ships were faster. So even though the first wave of ships were sent out first, they were the last to arrive and attack the home world.
I love science fiction books.