Whatever started the war was REALLY petty.
The war is to give the dragons an external enemy to fight against
- The dragons are prejudiced against each other: Light and fire dragons won't so much as give each other First Aid, nobody's batted an eye at the lightning dragons getting kicked out, and the earth dragons had to blackmail their way out of second-class citezinship. One queen a long time ago pointed at the tigers and told everyone that They want the dragons bogged down in internecine warfare so they can kill the survivors and take all their stuff. This of course does nothing to address the old hurts that made each subgroup hate each other, so they fester and lead to Teeth-Clenched Teamwork. It also leads to a situation where the war either ends in tiger victory or a draw: if the tigers win, they win. If the dragons win they immediately go back to declaring their respective subspecies the master race and killing each other.
- More or less Confirmed in strip 520.
Luminia is as powerful as he is because her DNA is more fresh.
Luminia is stated to have abnormally strong magic. Being the only dragon in recent memory to have half her genes be, erm, "from the tap," as it were, as opposed to the cloned junk everyone else is bred from (as per Word of God), she has none of the attendant problems. Dragons spend the first half of their lives as female and grow to be male
The dragons are shown to grow throughout their lives, like garden-variety reptiles. At 100, they are the size of humans. Male dragons are referred to as "Elders," implying they eventually reach their prodigious size later in life. It is also telling that the dragons haven't randomly produced any males over the course of the millennia-long war with the tigers, implying a change later in life. The setting takes place after the death of an advanced civilization.
Dragons have preserved some ancient technologies and techniques but have been backsliding due to a general laziness in fixing things for a 'oh we can just replace this with magic' mindset. Tigers have preserved cultural values, such as LGBT+ acceptance and electorate systems, but this has deteriorated into a matriarchy due to the gender imbalances.It took far too long for the outsiders to find this place again. Everything revolves around bringing them back into the fold - one way or another.