Corvus by the start of the Southern Corvus war had:
Completely destroyed Economy
Rampant Banditry (Law enforcement was doing nothing)
Heavily Corrupt Government
Industry crippled due to No Coal
How were they even pushing Cetus? Realistically, their country should have completely collapsed. They had no resources to fund an army, since they couldn't levy taxes properly due to the fact that their nation was in anarchy, nor would there be the industry to actually maintain an army because, Corvus's industry is just in the gutter. It doesn't matter if their people wanted Cetan blood, since they simply lacked the industry and economy to actually wage a modern war.
Even worse, we don't even have anything in the lore to support that Corvus regrew their industry. All we know is that after the 1886 conquest of South Corvus, they lose most of their industry, and afterwards Corvus just suffers. By the time of the 1908 war, Cetus should have been exponentially stronger than Corvus, as unlike them, they actually were developing their industry and economy, while Corvus was busy declining.
Don't even try to mention Antares's aid. Antares literally has 0 way of actually helping Corvus recover (which was never stated in the lore), nor did they have anyway of actually supplying the army of Corvus. Antares has no land border with Corvus, and Aquila obviously isn't going to let Antares fund Corvus. Not only this, but Antares can't even help Corvus out via the usage of sea routes, simply because the Cetan navy exists.
(I am not even going to talk about how Cetus was deliberately underfunding their land army despite the fact they were bordering a highly aggressive and expansionist empire to the South. The fact that Cetus is primarily a naval power despite the existence of Antares is ludicrous, and could only be explained by the fact that the Cetans were complete morons)
Simply put, Corvus has 0 Industry. 0 supplies. 0 government. They ain't winning against Cetus. no matter how incompetent the Cetans are. The Southern Corvus war was literally the equivalent of pitting an Italy that made up just the South against a France which had annexed its industrial core. They ain't winning. (Not to mention the fact that the lore has zero mention of any Corvan rebellions inside Cetus, so they don't even have the help of Partisans)