During the Oblivion Crisis, a faction known as the An-Xileel (an Argonian political faction comprised of several powerful clans, including the Scale-Song) rallied the Argonians against the Daedra after the Battle of Kvatch. When the Oblivion Gates opened in Black Marsh, the Daedra didn't so much flood into Argonia as the Argonians did into Oblivion. The natives fought and defeated the Daedra on their own turf, doing so with such success that the Dremora lieutenants actually closed their Gates themselves to prevent being overrun, an occurrence unheard of in the other provinces. Thus, the An-Xileel held Black Marsh until Martin Septim ended the Oblivion Crisis and closed the Gates forever. They also made Black Marsh the first province to secede from the Empire after the Crisis ended.
It is belived that cause of Argonian glorious victory over the Dremora is a tree called The Hist.
"The Hist gave his people life, form, purpose. It was the Hist who had seen through the shadows to the Oblivion crisis, who called all of the people back to the marsh, defeated the forces of Mehrunes Dagon, drove the Empire into the sea, and laid waste to their ancient enemies in Morrowind."
―An Elder Scrolls Novel: The Infernal City
So when they got warned about the invasion by sentient trees they started preparing for war in every way possible, men, training, armor weapons, potions.
Most of all, they where prepared for the fact that the gates would just pop up outside their cities. Even oblivion it self was working in their favoure as reptiles metabolism speeds up in hot conditions, so walking into oblivion would basically be like getting roided up and theyd be pretty frenzied. And after Dagon decided to have all portals to Black Marsh closed he said "Let's not go there again, it was a silly place."
Fun Fact about Argonians:
There are rumors of Argonians that have contracted a unique form of lycanthropy, and rather than becoming werewolves upon transformation, become creatures known as Werecrocodiles. They lurk in isolated swamps and spread the disease!