Other races?

[Spicy_Satoshi]

What conditions have to be fulfilled by the player in a campaign to command other races? As the Empire hero what to I have to do to have the Barbarians at my side?

[Aelies von Saladir]

The only thing you need to do is allying with the race.
However, the conditions for allying with different races are different.
For Barbarians, for example, you need to win the Barbarian Challenge at Daros Gap, defeating five chiftains in one combat.
Ssrathi can be allied with at Kalpaxolt, though you will have to oppose the Empire in order to Parlay with the Ssrathi. The Empire can be allied with by completing the Order of Selentine Quest chain. Minotaur need you to assamble a Hand of Sartek. Undead, Plague and Swarm can be parleyed with after helping them in the main quest line. Fey need the Challenge mission at Twilight Woods.
Some races like Knights and Wood Elves require you to repeat random missions over and over again, and some, like Daemons, just can't be allied with at all.

[Spicy_Satoshi]

OK I managed to gain some new allies, nevertheless I'm wondering how to convince The Dark Dwarves? I did what I could and in their homelands Khazdul and all I have is their friendship and access to a trader.

[Chronie]

Races if I recall correctly have a sworn enemy that you can attack to gain their trust. For Dark dwaves it is Dwarves, Plague and High Elf. Otherwise doing missions for them is usually the easiest way. Daemons will never ally with you.