In my universe, planets function like nations, and although travel is possible between them, the number of people who travel between planets is fairly low. Additionally, while broadcasts from other planets exist, most citizens of a planet do not generally preoccupy themselves with the status/news of other planets.
The current problem is that a nation, Nation X, has begun to perform secret takeovers of governments on other planets. This takeover project has been in place for around ten years. Methods of takeover have been to first gather information on governments and government leaders, then to either coerce the current leaders into bending to their will, or if failing, imprisoning or killing leaders and replacing them with individuals from Nation X. Citizens in nations that have been taken over are unaware of a takeover, and only notice that their travel and communications seem to be more restricted.
My question is, what would be a motivating factor for Nation X? Why would Nation X do this?