Basically, you need to research technologies that increases your Soldiering abilities. You can also mine those red diamond shaped Military Resource on the galactic map. Having a somewhat large population will also help you defend your planets. Don't build more than one farm because the more people living on the planet, the harder it will become to keep them happy.
Invasions also depends on your Soldiering abilities, and numbers as well. Put your troop transports into fleets before invading a planet.
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