The EU's 27 member states have shown they are able to reach agreement on issues such as climate change, immigration and trade. But the bigger states tend to carve out their own foreign policies rather than subscribing to an EU-wide agenda.
They sometimes differ on sensitive matters such as relations with Russia, Turkey and Cuba.
The result is often either a watered-down common position, agreement on less sensitive issues or backing for policies that are directly EU-related, such as the new diplomatic service.
Ashton is expected to focus attention on building the EU diplomatic corps, known as the External Action Service, which will have as many as 3,000 diplomats around the world and be overseen by her. The first appointments have sparked turf wars.
But she has also made clear that she wants the EU to have a bigger role in Middle East diplomacy -- working closely with the United States, Russia and the United Nations -- and to use a combination of aid and trade to strengthen overseas ties.
Analysts say it is essential that she chooses a few priorities and masters the brief, rather than trying to cover every issue the EU faces and getting lost in the melee.
"She was always saying that she was in a marathon and that being in a marathon she could pace herself," said Daniel Korski, a senior fellow at the European Council on Foreign Relations think tank, referring to Ashton's five-year mandate.
"Now she's beginning to realise that she's got to get up to speed quickly, look at who she's up against and start looking like a winner. She can sustain herself, but she also has to focus on issues where she can deliver something concrete."
(editing by Paul Taylor)