President Obama announced plans to greatly reduce the number of US forces in Afghanistan, to conclude combat missions in the country this year and to end the US military commitment by the end of 2016.
The president acknowledged that US forces had been in Afghanistan for longer than expected, but said they were now at the stage to plan to "finish the job we started."
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The Labour leader did not say whether he would maintain Britain's nuclear deterrent programme.
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.