Leading article: A lack of definition on the big issues

Tony Blair never passed up an opportunity to offer a grand moral justification for his foreign policy. But the style of his successor, Gordon Brown, has so far been quite different. Since he took over from Mr Blair in June, Mr Brown has resisted the urge to articulate an overarching rationale for his decision-making on international affairs. Remarkably, last night's Mansion House address was Mr Brown's first proper speech as Prime Minister on the subject.

We now know a little more about where Mr Brown wants to position Britain. Yet the speech left more questions unanswered than resolved. The meaning of the term "hard-headed internationalism", which Mr Brown was keen to push yesterday, is elusive. It seems intended to signal a shift away from the tarnished "liberal interventionism" associated with Mr Blair. But, beyond that, it is far from clear what it would mean.

Our only clues lie in the practical measures Mr Brown proposes. The Prime Minister stated that he wants to make international institutions "fit for purpose" in the 21st century. It is a ghastly phrase, but Mr Brown's indication that he will throw his weight behind the extension of the United Nations Security Council is welcome. It is ludicrous that this crucial decision-making forum is still dominated by the major powers of the 1945 era. The proposal for a UN "standby civilian force" to help rebuild societies damaged by conflict also sounds like a sensible way for richer nations to pool their resources for the greater global good.

The Prime Minister's praise for the closer relations between Angela Merkel, Nicolas Sarkozy and George Bush is interesting, too. Despite his trumpeting of Britain's US ties as "our most important bilateral relationship", Mr Brown seems to want to get away from the "Anglo-sphere" theme of the Blair years. Again, if that indicates a refreshed commitment to multilateralism from the Prime Minister, it is entirely welcome. The great challenges of the 21st century – climate change, trade reform, terrorism and nuclear proliferation – will be solved only though multilateral co-operation.

But there is a problem. Mr Brown has long been muted in his enthusiasm for co-operation with his fellow European governments – and he did nothing to dispel that impression yesterday. It is all very well for Mr Brown to put some subtle distance between Britain and the United States, but unless this is accompanied by a new willingness to work with EU governments and institutions, he risks isolating Britain on the international scene. That would hardly be in the national interest.

Another problem is that Mr Brown's speech does not enlighten us as to how the Prime Minister might respond if there were to be another breakdown in multilateralism as there was over Iraq. Most leaders are committed to international co-operation in principle. It is what they do when it breaks down that really matters. What would Mr Brown do if he had to choose between the US or the UN, as Mr Blair had to over Iraq?

That brings us, inevitably, to Iran. Mr Brown's language on the nuclear crisis has been less bellicose than America's, although his backing yesterday for tougher EU and UN sanctions will please Washington. But the real question is: what would Mr Brown do if Mr Bush decided to order the bombing of Iran's nuclear facilities before leaving office? Despite the disaster of Iraq, this is by no means an inconceivable prospect. Would the Prime Minister support this as a necessary "hard-headed" intervention, or would he condemn it as a breach of the principle of international co-operation? Yesterday's speech, for all its attempts to put some ideological flesh on the bones of Mr Brown's foreign policy agenda, left us none the wiser.