Several passes allow passage from one to the other, but most are very remote and seriously difficult to cross. There's only two passes that actually allow armies to transverse the mountains:

•  the Iron Pass in the south: once the trajectory of trade caravans, later becoming a battleground, which in the days of my story evolved into an abandoned wasteland only visited by reconnaissance units from both sides due to a successful treaty between the regional power centers on either side of the pass (this pass is named Sir Rẃidic in the High Tongue and Sgárni Privu in the Thieves' Tongue); and
• the Evening Glow Pass 200 miles to the north: a terrible, terrible route clinging to sheer cliffs and crossing treacherous glaciers. This second pass is still used for invasions from both sides, and abandoned every fall due to the dropping temperatures. Attempts have been made in the past to construct fortresses to withstand the horrible cold, but no one ever succeeded (this pass is named Sir Áinglaran in the High Tongue and Díshuda Privu in the Thieves' Tongue). 

East of the Dragon's Backbone lie the Freelands of Gaendyr. Once a vasal state of the waaaaay stronger Araniell, it managed to free itself in a desperate Independence War. Araniell was not really prepared to give up any land, but it didn't feel like a prolonged war either. By that time Araniell had grown fat and rich due to a prolonged peace, at least in most of its enormous territory. The Gaendyr region had already been plagued by invasions through both mountain passes, however. Araniell soon realised that it could either fight a rebel war it didn't feel like fighting, only to then continue to quell the yearly invasions from over the mountains... or just give up the war (after a magnificent show of force, naturally) and leave the people of Gaendyr to take care of the invasions. It pulled out all of its armies and posted them along the new border with Gaendyr, and watched as the newly formed country was weakened year after year by the invasions. Soon Araniell would be able to swoop in, pick up the pieces, and actually be thanked by the exhausted people of Gaendyr. 

Except, that never happened. Gaendyr turned into a scrawny little nation of warriors, squeezed between the immovable Araniell and the continuously invading peoples from Ilan, but it managed to survive. Barely, but it did. And eventually people from beyond the border began to send help too. Never in the shape of goods or gold, since these were strictly prohibited by the Araniell government, but help they did. It became a matter of pride among the West-Araniell houses to send troops to Gaendyr. No longer to beat the locals in the dust but to aid them in their wars against Ilan. Brave and bold as that move was, it wasn't entirely selfless. Araniell nobles living close to the Gaendyr border realised that they either helped Gaendyr defend its western border, or would someday face the Ilan invasions themselves. And mountain passes are far easier to defend than a 1000 mile long stretch of river...