Alien Technology
If your scientists are ever lucky enough to stumble upon alien technology, exploit the new technology as much as you can. Reverse engineer everything possible. Use this new knowledge all throughout your empire. This will serve two purposes. The first, and most obvious, is that you will be so much farther ahead than your enemies. They will still be operating in today’s technology, while you will have the power of the future. The second, and probably the most crucial, when the aliens who mistakenly introduced their technology to you returns, you will be prepared for them.
Since you have already learned everything possible from their technology, they will not have anything to offer in exchange for help or what have you. You will probably also have discovered their armament and have that spread across your empire, making it so much easier to take down and destroy the aliens. After all, the odds are that they are here on Earth for a more sinister plan than just passing by, or to collect their fallen comrades. They are more than likely scoping the place out, figuring out how best to destroy us all. If you can fight back with their own technology, they aren’t going to want to start anything, and may just leave everything alone.
If these aliens turn out to be giant autonomous robots, technology alone is not going to save us. They are going to be big piles of moving, heavy, metals. They can and will crush you like a bug. At this point, your only hope is that an opposing alien robot force comes to fight along side your armies. At the end of the battles, odds are they are going to want something as well. So just make sure you have something in reserve to take them out as well.