Leading article: The Prime Minister mounts his Afghan defence

Mr Brown tried to level with the public, but his timetable is unrealistic

Ever since his first speech as Prime Minister, Gordon Brown has tried to make a virtue of seriousness. He called on his trademark gravitas again yesterday in an appearance designed to allay the growing public doubts about the Afghan war. He tried to sound realistic, while also remaining positive. Squaring that circle was not easy.

The background to Mr Brown's speech was the accumulation of bad news from Afghanistan as the new political season opens. British casualties were higher in August than in any month since the mission began. The election, in whose name many of those lives were sacrificed, remains unresolved, and the turn-out in the south was "disappointing" – Mr Brown's own word.

Politically, Afghanistan now finds itself in a dangerous limbo. President Hamid Karzai may face a second round of voting next month; but charges of irregularities and outright fraud have multiplied, casting doubt on the credibility of the whole process. The arrival of a new US commander, General Stanley McChrystal, was expected to herald a sharp change in strategy – away from combat and back to state-building. But the components of that strategy seem still to be under discussion in Washington.

But Mr Brown's appearance, at London's premier defence strategy think-tank, had a more specific context, too: the resignation less than 24 hours earlier of an aide to the Defence Secretary. Not just any aide, either, but Eric Joyce, an MP known as a staunch government loyalist and one of very few with a military background. Mr Joyce's barbed resignation letter reiterated the ever more overt misgivings expressed not just by a sceptical public, but by the military.

His accusations set up the questions Mr Brown endeavoured to answer. Could Britain afford its current level of military engagement? Why was more not demanded of other Nato allies? How long would Britain's engagement last, and were the troops and their families given the spending priority they deserved? Mr Joyce also echoed a widespread doubt about the continued justification for the war: that our troops are fighting terrorism in Afghanistan to prevent it spreading here. Always specious, this argument has become more so, as ministers deliberately conflate Afghanistan and Pakistan.

Mr Brown, to his credit, offered a more grown-up exposition, as he also did answering questions. He paid tribute to Pakistan's recent efforts to expel the Taliban from the Swat valley and indicated that the tribal areas would be next. He acknowledged that the chief terrorist threat today came not from Afghanistan, but from Pakistan, while arguing – not unreasonably – that, to the extent the Pakistan army succeeded, al-Qa'ida and the Taliban could be displaced back to the Afghan side of the border. This is a more sophisticated account than ministers have produced hitherto, but the fallacy – that we are fighting terrorism there to stop it coming here – remains a fallacy for all that.

Mr Brown tried to defend his government against charges that the troops were poorly equipped, citing increases in spending and numbers of helicopters and armoured vehicles. Although he said the arrival of more and safer transport was imminent, however, it has still not actually arrived. And the troops have been in the field for the best part of eight years.

Nor did his prognosis carry complete conviction. The security and state-building objectives he emphasised depend on accelerated "Afghanisation" – the training of Afghan troops and police. But extrapolating a timetable for withdrawal from even the most optimistic scenario would leave British forces in place for at least another year – and progress in training so far does not suggest that the pace Mr Brown hopes for is realistic. Unless, of course, "Afghanisation" is a fig-leaf to cover a half-finished job that would allow withdrawal with a modicum of honour.

This was in many respects a back-to-basics speech, with many reminders of why Britain joined operations in Afghanistan in the first place: to prevent the re-establishment of terrorist bases; to build, or rebuild, an Afghan state, and to foster cultural values associated with moderate Islam as well as education for girls. But the rehearsal of these elementary objectives – shorn of the idealistic talk of democracy and freedom so beloved of George Bush – only served as a reminder of how far eight years of military intervention have fallen short.

Few could quarrel with such objectives. Harder to accept is Mr Brown's insistence that they are feasible, still less in the timeframe he mooted. Until the dust settles from the election and US strategy is clarified, the best of plans will be fruitless. For the time being there is no alternative but to keep a strong nerve and carry on.