Looking for a science fiction novel I read as a kid where a few hostile alien saucers show up and the US Air Force is trying to bring them down. It's too fast for any missiles/cannons to work, and finally the only way they hit one of them is with some sort of experimental laser carried inside a cargo plane. Later on, they detect a large alien fleet far outside the solar system, heading for Earth. Assuming this is an invasion fleet, they embark on a crash course to construct a gigantic laser (I believe it was gamma-ray based) to fire at the alien fleet before they get close enough to Earth to spread out.
I remember some sort of protester/activist tries to fly a small plane right down the barrel of the laser just as they are about to fire it and gets vaporized by the beam. The laser works and spreads out enough to hit the entire fleet at once. The radio chatter from the fleet increases after the laser shot, then gradually dies out to silence. The fleet of alien ships (presumably full of dead aliens) drifts aimlessly past Earth out into space. The book ends with one of the chief scientists/generals explaining to a bunch of world leaders how this is only the first step, how the aliens are likely to return, and how they need to expand, jump out into the solar system, build more giant lasers for defence on all the planets and moons. The world leaders are silent with horror, and the scene ends with everyone staring at a ticking clock on the wall.