Eurosceptics sometimes like to pretend that increased EU co-operation on defence might somehow conflict with or undermine NATO.

This myth is nicely blown out of the water (no pun intended) by NATO themselves. Here’s what the organisation's Secretary General, had to say about this in 2003:

“What is this row about European defence all about? Will the European Security and Defence Policy really damage NATO?

“My answer is an emphatic no. I have been as robust as anyone in my opposition to unnecessary duplication between NATO and the European Union. … But that does not mean I do not welcome a stronger European security and defence role, including the ability to conduct autonomous EU missions where NATO decides to stand aside. …

“I therefore welcome the agreement reached recently among the EU members on strengthening EDSP because it involves no unnecessary duplication. I am also reassured by the commitments to a strong Atlantic alliance, and to complementarity between NATO and the European Union, being made on all sides of the debate. …
“NATO and the European Union both have more than enough to do without a new round of theological nit-picking.”

Read the full article in the Nato Review journal.