The thing is though, distance in space is vast --- really vast. Travelling at speeds upwards of C would essentially be to cut away all communication with your home. The aliens could not get access to the experience of other invasions. In fact, they would lag behind technologically. Unless their spaceships are literal planets, they won't be able to improve on their technology during their travels. Therefore, the distance between them and us, in lightyears, dictate the difficulty of invading us.
There are two possibilities:
- They send their spaceships randomly in all directions
- They wait for signs of life and then react
In the first case, they won't know anything about us and will have much worse technology than in the second case. In the second case, there would be a long time period between the time we send our first signals and the time they arrive. We would be far technologically superior to today at the time of their arrival. Given that our planet is closer to the battlefield, we have the defenders advantage with regards to creating defences, quicker resupply and quicker implementation of new technology. In the second case, it would be nearly impossible for the aliens to take us down at all, given that technology is limited by the laws of physics which make their superior technology less dominant due to the nature of diminishing returns.
Therefore, the only case worth asking at the current moment is the first one. What if an aliens spaceship or small fleet enters our close proximity by chance and discovers our radio signals. In that case, they would come with superior technology but in low numbers, and they would fight our earthly defences. This battle would be interesting, and I think our Earthly defences would do better than you think. Spaceships are terrible for travelling in the air, in the ocean or on land. Bringing vehicles would be ineffective since our Earthly conditions require specific types of transportation. The spaceships could not land without being shot down by tactical nukes, and they could not stay in orbit for long before we put weapons on our rockets to fend them off. All in all I think an invasion is impossible, to be honest. Movies of alien invasions relly on breaking physical laws, which would be the equivalent of magic. In the real world, unpenetrable armour and alike doesn't exist.