The people, even the king, will rally to the Redoran cause. Who else would save their homes from the nords and orcs?
The Telvanni? No, they would ignore the invader.
The Hlaalu? No, they would do business with the invader.
The Indoril? The who?
The Dres? No army threatens them. Infact, if the pretender king wants their help, he'll rewrite the slave laws to their favor.