Why did Winston Churchill oppose Neville Chamberlain?

Winston Churchill opposed Neville Chamberlain mainly because of the events leading to the start of World War II and how the Allies were doing when World War II began. When Germany was taking land in Europe, Neville Chamberlain did little to stop this from happening. When Hitler wanted to annex the Sudetenland, Chamberlain and the leader of France, Edouard Daladier, met with Hitler and with Mussolini and signed the Munich Pact. This agreement allowed Hitler to annex the Sudetenland. These leaders tried to appease Hitler by giving into his demands, hoping that by doing so, it would stop Germany’s aggression. Chamberlain believed he had preserved peace by making this agreement. Winston Churchill was against this agreement as he didn't believe the policy of appeasement would be successful. When Hitler took over the rest of Czechoslovakia in 1939, it proved the policy of appeasement had failed.
Once World War II began after Germany’s invasion of Poland in 1939, the Allies did very poorly. In 1940, Germany rolled through Norway and Denmark. Then, Belgium, Luxembourg, and the Netherlands fell into German hands. At this point, Chamberlain lost a vote of confidence in the House of Commons. Winston Churchill became Prime Minister. He was known for his military background. He vowed that Britain would work hard to defeat Germany. He also laid out the British plans to resist Germany. When Germany tried to attack Great Britain, Winston Churchill said that Great Britain would never surrender.
https://www.history.com/this-day-in-history/churchill-becomes-prime-minister

https://www.history.com/this-day-in-history/munich-pact-signed

Winston Churchill opposed Neville Chamberlain because Churchill thought war with the Germans was inevitable and wanted the country to prepare for it by aggressively rearming.
From almost as soon as Hitler took power, Churchill was warning of the dangers of Germany rearming and forcing another war. Chamberlain, however, believed it was possible to appease Hitler by letting him have more territory in Europe. In Chamberlain's defense, Great Britain's people were still reeling from the costly blow of World War I, in which vast numbers of lives were lost. Nobody wanted another major war fewer than 20 years after the last one, which was burned into many people's memories as a horror.
Chamberlain hoped that giving Hitler the Sudetenland in Czechoslovakia would finally satisfy him and avert a war. As history has shown, this was wrong. Churchill was right in his assumption from 1933 onward that Britain needed to prepare for another war.