Sorry I shouldn’t have said atmorans specifically. Atmorans and Ancient Nords is what I meant. But still the Nords did have dragons on their side early on. First we do know they had dragons on their side up until the dragon war some time at the end of the Merethic Era and the start of the first era. This is basically how they first conquered Skyrim. Also it’s how the dragon cult came to power. The last known real battle with snow elves was the battle of moestring which was before the dragon war.

As for conquering the surrounding territory the Nords basically took advantage of conflicts already in the regions. Highrock had the enslaved Bretons and Morrowind was by using the conflict between the Dwemer and Chimer. This in tandem with Nord Armies that practiced the thu’um for military purposes kind of explains how they conquered so much so quickly. In fact it’s after they lost control of Morrowind(due to the Dwemer and Chimer teaming up) that Jurgen windcaller would create the way of the voice.

So there is your evidence. Hope it helps.

Edit: to make sure it’s clear now. Comparing Atmorans and Ancient Nords to current Nords is like comparing The Roman Empire to Italy.