Civ 5 - told I lack iron but have an iron mine?

I'm used to playing earlier versions of Civ. Playing Civ 5 and despite having an active iron mine, which is connected to my cities/trade network by roads, I'm told that I can't build any unit which requires iron, although I do already have one legion. Do you literally have to have an iron mine for EVERY SINGLE unit which requires iron?

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How do you get iron in Civ 5?

Iron is extracted via Mines, which makes it easy to access. Iron natural military training centers. Take advantage of this by constructing military training buildings like the Barracks in the city, as well.

How does iron work in Civ 5?

The discovery of iron, a metal much stronger than the bronze alloy used until that time, lays the groundwork for Iron Working and contemporary metallurgy. Its advantages in strength and durability allow it to quickly displace bronze and become the basis for tools and weapons for a very long time.

Where can I find iron in Civ 6?

Found on

Desert (Hills)
Grassland (Hills)
Plains (Hills) Tundra (Hills)

How to Mine Iron in Civilization 6 [+Iron Mining Basics]

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Mumtahina Tanni, Mumtahina Tanni, Jan van der Wolf, Mufid Majnun