"That is a fairly apt description, very well."
"Let me start at the beginning. Around five million years ago, a race we call the Ancients, who inhabited Earth at the time, suffered from a great plague, that consumed the whole galaxy. Some of them decided to leave and come here, in a city-ship called Atlantis. They built a network of stargates, and seeded some of the planets here with human life - we don't know exactly why, but that's a thing they did back in our galaxy as well. In this way, they lived for many generations, until around ten thousand years ago. In their confidence, the Ancients missed a growing threat - a species of insect found on one of the planets has been feeding on humans, and slowly transforming into a hybrid species called the Wraith. They can feed on human 'life-energy' - we don't understand the process exactly, visually it looks like people getting old at a rapid pace, but medically it's different. When the Ancients learned about them, they tried to exterminate them, but their first attempts were overconfident, they allowed some of their ships to be captured and Wraith had numerical advantage. The war raged on for hundrends of years, and eventually the Wraith controlled most of the galaxy, leaving Atlantis as the only remaining Ancient outpost, under constant siege. The Ancients abandoned the city, and used the stargate to get back to Earth."
"Since then, the Wraith were the dominant force in the galaxy. They treat humans as food source - allowing them to breed while the Wraith hybernate, and then performing Cullings, taking away most of the population. If any civilization develops to the point of threatening the Wraith, they are wiped out of existence. We believe your planet hasn't encountered them because they don't know you exist - your gate was buried for a long time, and they won't be trying to get to unreachable stargates many times."
"A few years ago, we uncovered an Ancient outpost on Earth, containing the location of Atlantis, and mounted an expedition to explore it and the galaxy. We had an encounter with the Wraith, in which they learned some information about Earth - importantly, because Earth has never been culled, its population is many times that of any planet known to them. Since then, they've been trying to reach it. Fortunately, they have neither the location of Earth, nor a ship capable of crossing the distance between galaxies. They have also made attempts to destroy Atlantis."
"And so, we are currently at war with the Wraith. Their numbers have been greately diminished since their war with the Ancients, but they are still a formidable foe. As long as location of your planet is a secret, they should not come for you, and I intend to guard it as safely as the location of Earth. We can't ask you for your support in the war, but we hope you will understand our position."