The first is correct. You are using the noun "union" as an adjective to describe the noun "climbdown". When using a noun in this way it cannot be possessive.
Compare it to this:
The Apple iPhone is a great gadget.
Apple's iPhone is a great gadget.
In the first sentence "Apple" is used as an adjective to describe the phone. In the second it is used as a noun to say which company owns this product.