One of the many factors which inhibit the development of a common foreign and security policy amongst member states is that this would be a direct encroachment on state sovereignty in an area which has been core to the idea of state control and the independence of states since Westphalia. For a more united foreign and security policy amongst member states, there would have to be new administrative bodies and measures of enforcement similar to that currently employed at the Council of Ministers. Although this could be done through sovereignty pooling rather than supranationalist mechanisms, there isn’t yet a strong enough social or political European identity to support these efforts, especially in recent years where domestic politics across Europe have swung towards Euroscepticism with newer Eastern European states lacking the same political identity as those of the West.
The second issue is that an effective EU foreign and security policy would also require a means to carry out this policy which belongs to the EU. Although Interpol does perform this function to a limited degree, foreign policy and more extensive security policy would require an EU army considerably larger than the current EU military arsenal, the EU Battlegroups. Currently under NATO protection, EU states would have to strike out on their own to establish this military and security force. Given that the current security umbrella in Europe is mostly paid for by the US, with major European economies failing to meet the NATO minimum defence expenditure target of 2% GDP, an additional European force without the breakdown of NATO is very unlikely. The EU’s current diplomatic capabilities would also have to be expanded upon which would again require more money and further integration.
Finally, the member states of the EU currently have quite distinct approaches to foreign policy and security making common policies highly unlikely. Whilst Germany and some smaller European states prefer to neglect defence spending, reject militarism and opt out of nuclear technology, countries like France and Britain use their nuclear arsenal to punch above their weight internationally, engaging in international intervention and airstrike missions repeatedly. Although the benefits which economic integration bring to member states is very clearly and agreed upon fairly unanimously, coordinating meaningful and yet cohesive foreign and security policies would be a very hard task with the current divisions which exist among EU members over defence.