Well previously, before Riot redid the Lore, there were the rune wars between Demacia and Noxus and also between their allies, Zaun and Piltover, which were so devastating that the rune magic was almost destroying the very continent of Runeterra itself.
To prevent that, a group of powerful mages, the "Summoners" ended the war by forcing a peace contract between those nations. If they wouldn't respect these contracts, the Summoner's would just use their insane magical powers to kill everyone who defies it - and they did several times. Instead, to settle political conflicts, every nation state would choose "champions", that would be controlled by the summoners to fight in the summoner's rift and other places. Summoners were also able to revive these champions whenever and create obstacles like minions and champions.
With not being able to fight Demacia anymore, Noxus needed someone else. Demacia, Noxus, Piltover and Zaun all were part of the contract, Shurima was still pretty much dead and Bandle City was very well hidden. They had to choose between Bilgewater, Freljord and Ionia. With the first two creating a lot of difficulties, either because of harsh terrain or political and economical ties that they didn't want to destroy, they chose Ionia, especially because they've never known war before. This especially was a driving factor for Noxus, they believed it to be a disgrace because they only believed in strength and might is right. In their eyes, Ionia was deserving of the invasion because they needed to be taught that not trying to achieve your strongest self will inevitably end in your own destruction.
Ionia didn't really care about the mainland up until this point, they were full of peaceful people minding their own business. When the Summoner's offered their protection Ionia refused because they didn't want the mainland to have influence and they haven't had wars ever before, so why would they need it? So basically being unprepared for the invasion and not being able to listen to the warnings of Demacia which couldn't intervene because they can't go to war against Noxus, they got hit hard. After reciving loss after loss and finally fighting back a bit under Karma's and Irelia's lead, they finally agreed to join the League of Legends.
This also meant that they had to settle their differences with Noxus like everyone else did, with a match on the Summoner's Rift. This actually did even take place irl, pro players had to battle it out and one side could only choose Noxian or Zaunite champions while the other side was picking Demacian, Piltover and Ionian champs. Ionia won and because of that, Ionian boots were added to the game.
Of course, they've killed the lore after all so it doesn't make sense now. Until they finally release enough lore for the whole universe to make sense again there also won't be any sense to it in the future. For example, the whole game concept doesn't make sense anymore and purely theoretically Runeterra should be done with by now since the Rune Wars never ended.
and possibly 
(he's not originally from Demacia after all.) and we really hate Noxus too!" Maybe theres an entry or something that i missed but this just doesn't make any sense to me.
lore you will notice how she had to travel to freljorh to go back to the big part of the continent, i assume that noxus is right between ionia an demacia, meaning you either cross the frozen land, or go throught pitlover zaun area (go around noxus territory) or go around the globe like magallanes, risking on the open ocean, infested with creatures and dark mist
from Ionia, When noxus attacked her village.
