Still did not answer the quest. Of course the only answer to the question "could we detect an outer space invasion fleet", is that depends upon whether they choose to remain hidden or wish to be seen. If they want to be seen, no problem, they will make it pretty bloody obvious they are there. Now if they want to remain hidden and wish to launch a debilitating strike say targeting, power stations, dams, bridges, communications hubs and all major transport hubs and then let chaos simply settle in a while, months even. Well, they could launch that attack from so far away, as none of the targets are moving that much and, even the dumbest warhead could be fired a year away from it target and allow gravity and orbital mechanics to finish it all off. So if they were invading and didn't want to be seen, pretty much it would all be over before we even knew they were there. Now if you think they should target the military first, you fail to appreciate how much chaos in the civilian population, failed energy, failed communications, food supplies, will impact the military and complete derail their deployment, if the chaos is simply allowed to occur with out any further strikes, with the public making huge demands on the military to solve day to day problems whilst no further attack is occurring. This would scatter and expose the military making it more accessible for long range strikes targeted at military assets. Then you get in closer range and selectively target all energy sources. Why rush the preliminaries, no reinforcements from somewhere, just a population to be taken back to the stone age. Of course why bother, there are more resources out in our Oort cloud, already 'mined' and out of the 'gravity well', so pointless to go grubbing around on the planet for them. The best asset is likely to comedy show itself, watch mankind psychopathically eat itself alive, watch the cranky short hair, crested rock throwing monkey people, torture and kill each other, they could screen that to the rest of the galaxy. We have already been invaded by greed driven stupidity and unless we put an end to it, it will put an end to us and any invaders just need to sit back watch the show and laugh.