This has been an especially unhappy summer for the United Nations," Harvard historian Niall Ferguson observed in the Los Angeles Times recently and who could disagree?

With its mission in Lebanon unable to control Hezbollah, its blue-helmeted observers on the southern border blown away by Israeli shells and its role in the latest Mideast crisis being worked over in that boxing ring known as the Security Council, the UN seems to have fallen far short of its original, 1945 mission to "save succeeding generations from the scourge of war".

Even the ceasefire that was finally negotiated looks incomplete and liable to fragment in the very near future.

So, is the UN good for anything? Amid personnel scandals, the oil-for-food fiasco and a constant barrage of neoconservative attacks, that's a fair question.

And anyone who holds a belief in the value of the international organisation should be ready and willing to answer it. The easy way out would be to point to the many instances in which UN representatives have done well.

But that would seem an evasion to the many observers who focus on the grinding struggles along Israel's borders or the war on terrorism.

So, any defence of the UN has to be very careful in explaining what the organisation can do and what it cannot.

The first truism is that the United Nations is not, and never has been, a large and centralised actor in world affairs.

It is, if you like, a sort of holding company, with governments as the shareholders and with some of those shareholders the five permanent, veto-wielding members of the Security Council having much more voting power than others.

Reservations

True, all signatories to the UN Charter agree to surrender some sovereignty, but always with reservations. Very little about the UN's peacekeeping powers is clear-cut.

Everything depends on the circumstances. This is why it has been and will be so difficult for the world body to bring lasting peace to Lebanon.

First, the five permanent veto members have to agree on what is to be done or, at least, not disagree.

Second, UN peacekeepers may be very constrained. The resolution authorising "all necessary action" is actually quite vague in its instructions about where and when force might be used by blue-helmeted troops if Hezbollah-Israeli fighting resumes.

Above all, even a large UN operation would find it impossible to crush Hezbollah, let alone the Israeli Defence Forces, should either side resume belligerencies. And the subtle, cynical truism?

The UN is a scapegoat for the failures of the leading governments to agree or to act. Yet, as every secretary-general has discovered, it remains convenient for the major powers to blame the world body for their own failures to cooperate.

And this, as some weary UN officials suggest, may be one of the organisation's most important roles for if there was not a UN to blame for inaction in the face of disaster, then the finger might point directly at the various governments themselves.

This suggests, then, the limitations of what the UN may be able to achieve in bringing stability around Israel's battered borders. We certainly should not expect too much.

International aid will pour into Lebanon and various UN agencies will assist in the rebuilding, under the aegis of a UNIFIL force beefed up by much larger numbers of blue helmets.

If the ceasefire holds, this operation could go on for years, even decades. Why, future historians might even term it a success!

Blame

But if radical Muslim splinter groups resume firing rockets and Israel responds in a sledgehammer way; if the permanent veto members quarrel about who is to blame, then the many promising UN activities on the ground in Lebanon will end, international staff will be withdrawn and the downward spiral will resume.

Still, there will be one consolation. We will all be able to blame the UN for being ineffectual, weak-toothed, anti-Israel or anti-Arab and thus of no good to the world community.

It really is quite convenient that we possess such a scapegoat. If we didn't, we would have to invent one.

Paul Kennedy is a professor of history at Yale University.