This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.
Q: Napoleon III, since the Crimean War Britain's closest ally, visited London in April 1855, and from 17 to 28 August the same year Victoria and Albert returned the visit. Napoleon III met the couple at Dunkirk and accompanied them to Paris. They visited the Exposition Universelle (a successor to Albert's 1851 brainchild the Great Exhibition) and Napoleon I's tomb at Les Invalides (to which his remains had only been returned in 1840), and were guests of honour at a 1,200-guest ball at the Palace of Versailles.
A:
who was britains closest ally since the Crimean war?