Foster was the British Minister Plenipotentiary in Washington from August 1811 where he failed to resolve the long-standing dispute over the impressment of American seamen into the British navy as part of the war against Napoleonic France. America was neutral in the conflict and the service of American seamen in the Royal navy compromised that neutrality.

The tragedy of the situation is that by the time the Americans declared war on June 18th 1812, unkbeknown to them, the dispute over which they went to war no longer existed - the contentious orders had been withdrawn in London just two days previously.

The Americans prepared to invade Canada but their offensive was out-maneuvered with british troops marching southwards to burn the Capitol and destroy most of the Library of Congress in 1813. Foster had no alternative but to return home in disgrace (he was later sent to Denmark and Turin) and the Americans won the war at New orleans in 1815.

In 1845, the former MP and diplomat bought Brownsea Island in Dorset's Poole Harbour for £14,000. His retirement on the island was not a happy one for his health was failing and he suffered from deep bouts of depression. Following a severe illness, he committed suicide at the castle by cutting his own throat on August 1^st 1848.