While his watch has included 9/11 and wars in Afghanistan and Iraq, the straight-talking Scot's disappointment stems from a past conflict: the Bosnian war, and specifically alleged war criminals Radovan Karadzic and Ratko Mladic.

"What's disappointing is that Karadzic and Mladic are not in the Hague," he says, referring to NATO troops' failure to capture the Bosnian Serb chiefs and hand them over to the Hague-based UN tribunal for warcrimes in ex-Yugoslavia.

They will be eventually, he says. "But I'm sorry that during my time we were not able to pin them down," he said over lunch, as he prepared to leave NATO's Brussels headquarters for the last time.

"It's not just our responsibilty, it's shared by countries in the region... but until they are dealt with the Balkans won't be 100 percent secure," he adds.

When he walks out of NATO's door he will leave behind him a transformed organization: not least because this year it took command, in Afghanistan, of its first mission ever outside of Europe.

And the Alliance, which took over the International Security Assistance Force (ISAF) command in August, is even being tipped to succeed the US eventually in overall command of operations in Afghanistan.

A decision on that may be made at a NATO summit in Istanbul next June, when the bloc will boost its size to 26 countries with the addition of seven ex-communist states.

The same achievements cannot yet be said of Iraq, the other post 9/11 theatre of war.

In fact the Iraq conflict plunged NATO into the worst crises in its 54-year history, after three anti-war member states -- France, Germany and Belgium -- blocked it from coming to the aid of fellow member Turkey.

Robertson freely admits that the February crisis was "damaging," but denied that it was a near-death experience for the West's Cold War era military arm. "I wouldn't go that far. That would have been over-dramatic," he says.

Although he will remain NATO secretary general until December 31, Robertson is due on Wednesday to chair his last North Atlantic Council (NAC), the highest decision-making body of the Alliance, before saying his farewells to Brussels.

A lot of water has passed under the bridge since the former trade unionist, close to British Prime Minister Tony Blair, was catapulted into NATO after his impressive performance as defence secretary during the 1999 Kosovo war.

Over the last four years the pugnacious defender of the Alliance's London-Washington axis has championed its modernization and transformation from Cold War warhorse to post 9/11 global security alliance.

Under his mandate NATO, which had only just waged its first-ever all-out war campaign over Kosovo, has in particular forged a historic rapprochement with its former Soviet-era foe Russia.

"He has had to tackle difficult situations," said one diplomat, citing the Iraq crisis, or the aftermath of 9/11 when, although NATO had for the first time ever invoked Article Five of its founding Washington Treaty, it was effectively sidelined by Washington.

In February Robertson -- full title Lord Robertson of Port Ellen -- starts a new career in the private sector, becoming deputy head of Britain's Cable and Wireless plc. His successor, former Dutch foreign minister Jaap de Hoop Scheffer, will take up his job on January 5.

Asked about his biggest frustration over the last four years, Robertson is clear: "I'm frustrated most by the fact we have so many soldiers on paper and so few that can actually be deployed.

But he said: "I see signs of change there."

WAR.WIRE