The air nomads are a peaceful people, who believe(d) in isolation and non-violence. They had no quarrels with any of the other elemental groups, and had no means with which to defend themselves. The আগুন nations attack was sudden and unexpected, which is why the air nomads were so defenseless. It has nothing to do with weakness.
Did আপনি pay attention to the series at all? There's no weaker অথবা stronger element of the 4 main bending styles. Doing Sozin's comet the আগুন Nation was extremely strong thanks to that. Other than that they couldn't take down the Air Nomads দ্বারা themselves. And if আপনি realized they needed the comet to come around again to fully take over the world. It just so happens Aang and everyone was there to stop it this time around.
There Is No Weaker অথবা Strongest Element. Each Of Them Has It's Unique Characteristics. The Element Of Air Doesn't Specialize In Attack But In Evasion And Speed. The Air Nomads Are Pacifists. They Will Not Fight To Kill, Only To Protect Themselves And Those Around Them. Of Course That Doesn't Mean That An Airbender Cannot Win Against Another Type Of Bender. During The আগুন Nations Invasion On The Southern Air Temple, Monk Gyatso, An Experienced Airbender Was Able To Take Down Hordes Of Comet-Enhanced আগুন Nation Soldiers Before His Demise. There Are Also Many Other Examples Of How Powerful The Element Of Air Can Be. For Example অবতার Aang And If আপনি Have Watched The Legend Of Korra, Tenzin And Zaheer. Finally About The আগুন Nation's Invasion Don't Forget That Firebenders Had Their Abilities Greatly Enhanced দ্বারা The Sozin's Comet At That Time !!!!