What is a 'Failing State', and when is it a security threat?

The European Security Strategy identifies ‘state failure’ as one of the ‘key threats’ confronting Europe. This is one point of convergence with the 2002 US National Security Strategy, which claimed that ‘failing states’ were now more of a threat to the US than ‘conquering states’. However, implicitly distancing itself from the US, the European Security Strategy recognises that ‘none of the new threats is purely military; nor can [they] be tackled by purely military means.’