Wikipedia states that Perfect Forward Secrecy "is a feature of specific key agreement protocols that gives assurances that session keys will not be compromised even if the private key of the server is compromised".
In the example on Wikipedia two partner us a "key exchange algorithm such as Diffie–Hellman, to securely agree on an ephemeral session key. They use the [private] keys [...] only to authenticate one another during this process".
I understand that by using a ephemeral session key, if the private key is compromised the session is still secure. What I can not understand is, if you capture all traffic, can you not encrypt the key exchange part of the session and therefore gain the session key which then can be used to encrypt the session? Is this secured by the Diffie-Hellman key exchange?