Emm, could you please specify which game and which campaign? (If you launch Twilight of Arnor, you can play all three campaigns, but the Dreadlords campaign becomes very hard due to the changed combat mechanics.)
The campaigns in Galactic Civilizations are NOT to teach you the game, but to allow you to follow the history of this fictional universe, when civilizations had their hardest times. For the campaigns forget 'fair chance'. Apart from the videos (that you should watch), there is no interactive tutorial.
If the scenario goal is to capture a specific planet and your ally captures it, try to negotiate for that planet right after the takeover (the lower the population the cheaper is to buy the planet).