About five years after the incident at Red Crossing, the Dalish went on the offensive with great success. It was only after the elves conquered Montsimmard and were laying siege to Val Royeaux that the Exalted March was declared. It's like if the Pope at Avignon/Rome was besieged by the Lithuanians, after all of France/Holy Roman Empire's armies have been defeated in the field. It's a collective holy-****-this-is-a-crisis-on-so-many-levels scenario, and all of Thedas rushes to defend their head of religion, the capital of one of the most powerful empires, maintain the status quo, and throw down the heathens and the racially 'inferior' elves all in one.
So I think Red Crossing was just the casus belli for the initial conflict. Once the seat of the Chantry was threatened, it was expanded into an Exalted March. It was only when they were facing the full might of a continent that the Dalish lost, and even then, it took ten years. To me, that speaks of the Dales having been a very potent force, which is interesting considering they apparently only had Halamshiral and various small settlements, and their numbers were still lacking.
I'm pretty sure it wasn't five years between RC and Exalted March (more like a year) and Dalish forces were advancing towards Val Royeaux rather than already at the gates, but other than that you're correct.
Though in DA:I we're told that Orlais was supposedly the only country that actually put their forces on the field for Exalted March. It's not unthinkable that Mother Giselle is simply wrong about that, but I prefer thinking about such occurrences more along the lines of "well, technically true, but". The "but" in this case would be that it doesn't really take official involvement of a country to send sizable reinforrcements, I imagine that quite a few Andrastian noble houses actually fielded some soldiers and went on the crusade of their own volition. Combined with remaining Orlesian forces bolstered by new recruits (it's just easier to have people fighting when it's more than "king said we should" but more like "it's a holy war, guys, Elves are coming to destroy our faith").
So while to my knowledge/interpretation it wasn't really "Dales vs the whole continent", neither was it "Dales vs Orlais only" thing mother Giselle tries to sell us. The Dalish could've been driven back by Orlais but it's highly unlikely that they could lose as decisively as they did - the result definitely suggests involvement of additional forces, official or not. You just don't lose two of your major cities to an opponent you're actually able to crush decisively enough to actually gain total control of his territory to extent sufficient to start some large-scale relocation programs...
Though I do also believe Dalish had much more strength even after the fall than they are given credit for; my pet theory is that they expected Exalted March to end, people other than bloodied Orlesian military to go home with Orlais/Chantry setting up an occupation of Dales. That's why Dalish didn't scatter immediately, they retreated east, crushed Alamarri opposition in the area and waited for opportunity to go back, incite an uprising and re-capture Dales. Orlais would have much harder time to call for another Exalted March if elves were just trying to re-take the Dales. Orlais (and Chantry) didn't settle for occupation, however - they capitalized on advantage they had at the moment and went out of their way to actually destroy Dalish people - they uprooted the settlements, shipped elves away from the country (and presumably brought in human settlers in their place), all that still under guise of Exalted March, while also handing out gained land to nobles that lent their support (side effect of this process being keeping them and their forces in Dales)... In the end, before the March dissolved enough so that remaining forces could make their move, elves in the Dales themselves were simply too weak to begin an uprising, some likely blaming their rulers and army for all the injustice they suffered from human hands... Successful insurrection was not going to happen and majority of Dalish gave up on it in favor of some ambiguous waiting for better opportunity they kinda perpetuate to this very day.
And sorry for walltexting my private theories, I somehow fail to stop myself...
Repeatedly.