What was the reason for the Crimean war in 1854?

The war, which claimed an estimated 650,000 lives, pitted Britain, France, Turkey and Sardinia against Russia, whose ruler, Czar Nicholas I, was attempting to expand his influence over the Middle East and the eastern Mediterranean at the expense of the declining Ottoman Empire. The British and French, in turn, saw Nicholas' power grab as a danger to their trade routes, and were determined to stop him.

https://www.history.com/topics/british-history/crimean-war
https://www.historic-uk.com/HistoryUK/HistoryofBritain/Causes-of-The-Crimean-War/