The war between Huaxia and Twilight seemed sudden but it wasn't.
The Federation had suppressed their fury for more than 20 years. 20 years ago, they didn't have enough evidence to prosecute Twilight. If they did, this war might have started a long time ago.
As for the Twilight Empire, it was obvious that the emperor and the members of the cabinet wouldn't accept the proposal. They would definitely resist. Thus, the Federation's proposal was actually forcing the Twilight Empire to go to war with them.
Only Caesar had the power to say something in the whole situation. However, Caesar just remained quiet. They watched silently as the tension between the two countries increased until the war was imminent.
It was an all out war so god-class mecha masters were able to participate in the war
If there weren't any allies willing to help Twilight, no one was even willing to bet they could win.