My experience (being general of Riombara during most of the time)
Phase 1: All daimons attack Enweil:
We managed to get /all/ human realms (with the exception of Melhed and IVF) to fight jointly in these battles. The humans were outnumbered most of the time - and th daimons had the advantage of being able to recruit close to the battlefields. Still, we managed to kill several daimon armies this way.
During that time Riombara did send their whole army to aid Enweil several times. The only ones staying behind were inactives. This did result in monsters hurting us quite a bit...and once a daimon army did burn their way from Fwuvoghor through the isles, past Grehk and on towards Lopa, costing us around 20.000 peasants - while we fought in Enweil.
Phase 2: daimons have near destroyed Enweil, head north.
Phase 3: the horrors appear and attack the capitals.
I think we can agree that defending capitals in that phase was the only logical choice. Riombara lost Grehk, then MotS did take Fheuvenehm, then Enweilios. Riombara did try to send aid (again, the whole army) far towards IVF. We also sent our army to defend Enweilios but did arrive a halfturn too late.
Phase 4: daimons change tactics - the do not attack capitals anymore but concentrate on outlying regions.
This phase did start after the daimons did take Enweilios. After losing Enweilios we thought that MotS would head right for Rines - thus we scrambled to defend it. Once it became clear that he was not heading straight towards it we - again - sent our whole army to fight him in Rumannen. This is equivalent of marching across half the continent. Again leaving our capital completely uncovered - if any other daimon would have marched out of the blight /anywhere/ we'd have lost Rines - and this would have meant the practical end for Riombara. Thus we did take quite a risk.
After Rumannen we continued to march out to engage MotS actively: Mio Dupaki and finally Ardmore where we killed him (the 4th full battle we fought against him, always seeking him out). The following daimons were all engaged and killed in the field - with the exception of one of the portal guardians.
So - during the entire time of the invasion from the heavy allied fighting in Enweil until the end, the only time we did concentrate to defend Rines was for 2 weeks during the time horrors attacked capitals and during the final assualt of the portal guardians. We did risk losing first Grehk, then Rines near the entire time.