The Afghanistan war, the longest overseas conflict in American history, has cost the US taxpayer nearly $1tn and will cost several hundred billion dollars more after it officially ends this month, according to Financial Times calculations and independent researchers.

About 80 per cent of the spending on the Afghanistan conflict has taken place during the presidency of Barack Obama, who sharply increased US military presence in the country after taking office in 2009. The enormous bill for the 13-year conflict, which has never been detailed by the government, is an important factor in the broader reluctance among the US public and the Obama administration to intervene militarily in other parts of the world — including sending troops back to Iraq.

John Sopko, the government’s special inspector- general for Afghanistan, whose organisation monitors the more than $100bn spent on reconstruction projects in the country, said that “billions of dollars” had been wasted or stolen on projects that made little sense. “We simply cannot lose this amount of money again,” he said. “The American people will not put up with it.”