There's a big big difference between a soldier killing combatants on the battlefield and a person who knowingly prolongs a war for personal gain. One is justifiable as an ugly necessity the other isn't.
Briala is working for a better future for all elves, not her personal gain. It's about freeing elven men, women, and children. I think that cause is justified.
So you have a weakened Orlais and an untrained nation of elves against a well trained and prepared invading army at full strength?...Yeah they'll be crushed.
Assuming Nevarra is unhindered by the veil tears, which may extend beyond the borders of Ferelden and Orlais.