Space Empires Evil Ruler Handbook.

[dogscoff]

As the enemy is going on his merry way from your supreme court, ensure that a suitable "accident" befalls him. For example, a meteor falling from the sky escapes your detection and crushes the car he's travelling in. Whatever you do, make sure he's dead, and can never oppose you again

Hell no. Just have him killed and the cloned/ replicated/ impersonated by a shapeshifting alien loyal only to you/ replaced by a robotic double/ skinned and turned into a suit that you can wear to look just like him.

Then, with careful media management, you can convince his people that he is still alive and in charge, and, through some brilliant public negotiations (in which you control both sides of the debate) your two nations can come to some kind of 'peaceful co-existance' (ie, you absorb his empire completely.)