The Earthmen had been taken on a carefully conducted tour of the Rats' home planet, and the captain of the Earth ship—who had gone down in history as "Sucker" Johnston—was convinced that the Rats meant no harm, and agreed to lead a Rat ship back to Earth. If the Rats had struck then, there would never have been a Rat-Human War. It would have been over before it started.
But the Rats were too proud of their superiority. Earth was too far away to bother them for the moment; it wasn't in their line of conquest just yet. In another fifty years, the planet would be ready for picking off.
Earth had no idea that the Rats were so widespread. They had taken and colonized over thirty planets, completely destroying the indigenous intelligent races that had existed on five of them.
It wasn't just pride that had made the Rats decide to wait before hitting Earth; there was a certain amount of prudence, too. None of the other races they had met had developed space travel; the Earthmen might be a little tougher to beat. Not that there was any doubt of the outcome, as far as they were concerned—but why take chances?
But, while the Rats had fooled "Sucker" Johnston and some of his officers, the majority of the crew knew better. Rat crewmen were little short of slaves, and the Rats made the mistake of assuming that the Earth crewmen were the same. They hadn't tried to impress the crewmen as they had the officers. When the interrogation officers on Earth questioned the crew of the Earth ship, they, too, became suspicious. Johnston's optimistic attitude just didn't jibe with the facts.
So, while the Rat officers were having the red carpet rolled out for them, Earth Intelligence went to work. Several presumably awe-stricken men were allowed to take a conducted tour of the Rat ship. After all, why not? The Twentieth Century Russians probably wouldn't have minded showing their rocket plants to an American of Captain John Smith's time, either.
But there's a difference. Earth's government knew Earth was being threatened, and they knew they had to get as many facts as they could. They were also aware of the fact that if you know a thing can be done, then you will eventually find a way to do it.
During the next fifty years, Earth learned more than it had during the previous hundred. The race expanded, secretly, moving out to other planets in that sector of the galaxy. And they worked to catch up with the Rats.
They didn't make it, of course. When, after fifty years of presumably peaceful—but highly limited—contact, the Rats hit Earth, they found out one thing. That the mass and energy of a planet armed with the proper weapons can not be out-classed by any conceivable concentration of spaceships.
Throwing rocks at an army armed with machine guns may seem futile, but if you hit them with an avalanche, they'll go under. The Rats lost three-quarters of their fleet to planet-based guns and had to go home to bandage their wounds.