Resources: The Parthians
Historically, the Parthians stood between China and Italy. They controlled the middle of the Silk Road. Recently marina_bonomi and I got to talking about them, regarding their role in historic-time and allied-time. So I looked up some of the resources they had.
Their main strength and main weakness is the same thing, plain old geopolitics. Because they're in the middle, they can obstruct trade, travel, and communication. If it works, they've got a bottleneck. But that also leaves them vulnerable to attack from both sides. This linear version of the Silk Road shows one of the main routes. This version of the Silk Road shows it as a complex web, allowing people to get around obstacles.
The army fielded by the Parthian Empire had a combination of light and heavy (the cataphracts) cavalry which the Romans found difficult to defeat with their own reliance on infantry. The Mongols were skilled horsemen, so aid from that direction is possible.
The Parthians were pretty well set for natural resources. They had a broad belt of agriculture, not very useful for warfare; but also some mineral resources such as iron, copper, silver, and gold. Maybe a little timber, not much. It doesn't look like they had much exotic stuff of their own. The Parthian Empire is also on top of some major oil and natural gas fields, which could impact the later part of our series if the political structures shift. On this map, oil fields are marked in black, natural gas in red.
So, we have two strong allies -- China and Italy -- separated by a rival who controls vital territory right in the middle of a main travel route. That seems promising for plot conflict and for changes in historic flow as the alliance impacts the outcome of events.