I believe if an ETI decided to attack us, we'd be screwed. Here's why.<br /><br />Consider technology growth. The law of accelerating returns (as described by Moore's law) shows that the computing power available for $1000 doubles every 18 months. It is, in fact, actually speeding up every year, but let's ignore that for now. Given a doubling ever year and a half, in only one hundred years we will have computers that are over 117,000,000,000,000,000,000 (117 million billion) times more powerful than today. Imagine the kinds of technology that will be available to us with that much power available to the average person for only $1000!<br /><br />Now consider space flight. It only makes sense to send a spaceship on a long journey when the time for that journey is shorter than the time it would take to design, build, and travel on the next generation (faster) spaceship. If I recall correctly, it will take the Voyager space probe over 25,000 years to reach the distance of Alpha Centauri, our closest neighbor. Clearly this would not be a good ship on which to travel, so we will wait to send actual people or robots until we have a much faster means of getting there. Therefore, I surmise that as soon as a race has the ability to travel interstellar distances at all, (let alone for conquering purposes) they will be far advanced of us in technology. If on conquering expeditions, they would need to be even more advanced.<br /><br />Consider the age of the universe. Many leading experts in the field of exobiology postulate there could be civilizations thousands, millions or even billions of years in advance of us. If an ETI were only 100 years ahead of us, they would hold huge advantages in any arena imaginable. If a conquering race a thousand years ahead of us came our way, we wouldn't stand even the slightest chance.