Napoleon at first had a pro-British foreign policy, and was eager not to displease the British government whose friendship he saw as important to France. After a brief threat of an invasion of Britain in 1851, France and Britain cooperated in the 1850s, with an alliance in the Crimean War , and a major trade treaty in 1860. The Cobden–Chevalier Treaty Of 1860 lowered tariffs in each direction, and began the British practice of encouraging lower tariffs across Europe, and using most favored nation treaties. However Britain viewed the Second Empire with increasing distrust, especially as the emperor built up his navy, expanded his empire and took up a more active foreign policy.