The Allied invasion of Normandy on June 6, 1944 is considered one of the most consequential developments of World War II and instrumental in defeating the Axis powers. 156,000 troops landed on the beach as part of Operation Overlord, but before they would carry out the liberation of Western Europe, British Prime Minister Winston Churchill and U.S. President Franklin D. Roosevelt spent months debating the viability of such a risky mission.

Whether Winston Churchill opposed and argued against D-Day has become a subject of debate, with some accounts asserting he did his best to postpone or cancel the invasion. There is also debate over whether U.S. President Franklin D. Roosevelt kept moving the date for a cross-channel invasion. The truth is: it’s more complicated than that.

Evidence shows that both Churchill and Roosevelt were early supporters of some version of a “second front” in Europe. However, from early 1942 until mid-1944 both men fluctuated in their levels of support. American military preparedness, ever-changing warfront status, pressure from Soviet leader Joseph Stalin, weather conditions, and the varying definitions of a “second front” caused both leaders to frequently recalibrate their plans for D-Day.

Here is how both men came to agree on carrying out one of the most ambitious, and risky, military operations in history.

Hampered by a reluctant Congress, President Roosevelt could only send war supplies and lend military ships to Great Britain once England entered the war in 1939. Churchill understood how the American democracy worked, but continued to lobby Roosevelt for boots on the ground.

On December 8, 1941, the day after Japan’s attack on Pearl Harbor, the United States congress declared war. Within three days, Germany engaged the United States, honoring its agreement with Japan.

From the beginning, Roosevelt and Churchill knew that a massive invasion of mainland Europe was mandatory for a total and unconditional surrender of Nazi Germany. They believed the sooner the counter-attack, the better. Premier Joseph Stalin also pressed for a second front in the west to ease pressure on the Soviet army in the east.

However, each leader faced internal pressures and evolving circumstances within their own countries that forced them to consider alternative strategies and repeatedly push back the date for D-Day.

VIDEO: A Key to Victory

D-Day raised-relief map of Utah Beach prepared U.S. troops for the 1944 invasion of Normandy.

In March 1942, Roosevelt informed Churchill that the demands of the Pacific war reduced the opportunity for an invasion of Europe by that summer. The U.S. couldn’t muster the number of ships and landing craft needed for an operation of such dramatic scale, which military experts estimated would take a minimum 400,000 troops. However, the next month, Roosevelt sent two of his top advisors, General George Marshall and Harry Hopkins, to London to meet with Churchill.

They proposed “Operation Sledgehammer,” a plan to seize ports along France’s northwest coast and then conduct a major invasion in the spring of 1943. British military advisors believed they couldn’t gather enough resources in time, and the plan would end in disaster. In spite of this, Churchill seemed to give his endorsement to the plan in an April 17 message to Roosevelt.

Meanwhile, Soviet pressure for a second front intensified. In late May, 1942, Soviet foreign minister Vyachelsalv Molotov met with Roosevelt, urging for a way to relieve pressure from the eastern front by the end of the year.

Roosevelt and Churchill soon discussed a different approach: instead of Western Europe, they would invade Northwest Africa. If successful, “Operation Torch,” as it was known, would relieve pressure on British forces defending the Suez Canal, improve standing with the Soviets, and lift morale in both the U.S. and Britain.

Roosevelt knew that the effort would rule out any European second front for 1942 and probably 1943, but still agreed. He knew it was clear that it would take months to prepare for a large-scale attack, and due to impending weather along the English Channel, they simply wouldn’t have enough time.

Operation Torch began in November 8, 1942. It was the first major operation U.S. troops took against the German army.

Three days later, Churchill suggested to Roosevelt that their military commanders explore the possibilities of attacking Sicily and then Italy. Roosevelt expressed concern over U.S. involvement in the Mediterranean, but wanted to confront Germany as soon as possible while resources were available. He also worried, as did Churchill, that without military support, Stalin might call for a peace agreement with Hitler.

Roosevelt and Churchill met in Casablanca in February 1943 and agreed to an invasion of Sicily beginning in July. Churchill’s peripheral strategy of attacking the “soft underbelly” of Europe offered several advantages, namely knocking Italy out of the war.

The invasion of northern France was once again delayed.

Both the North Africa and Italian campaigns were lengthier and costlier than expected. When Churchill met with Roosevelt and Stalin in December, 1943, the defeats at Dunkirk, Anzio, and Salerno were likely on his mind.

Upon hearing that top Allied generals Dwight Eisenhower and Bernard Montgomery wanted a massive landing in Normandy, Churchill offered several alternatives to a massive full-frontal invasion, which Stalin insisted was the only action. Churchill sensed that the Soviets and Americans had already decided on the invasion, dubbed “Operation Overlord,” and shifted to support.

In January 1944, Churchill wrote to Stalin declaring everything was going “full blast for ‘Overlord.’” As late as April, Churchill continued to express some reservation telling one advisor that, “This battle has been forced upon us by the Russians and the United States military authorities.” By May, just before the planned invasion, he told the dominion of prime ministers of the Commonwealth that he favored Operation Overlord, though he never offered full-throated support and considered it a secondary strategy to the “soft underbelly” approach.

Churchill knew the Allies weren’t prepared for a large-scale invasion and developed alternative plans to defeat Nazi Germany. Roosevelt was hindered by war with Japan, so he supported North Africa and Italian campaigns to make progress against Germany.

Though the Allies weren’t prepared throughout 1943 and 1944, the invasion grew more and more important for victory.

The D-Day invasion took place on June 6, 1944. On that day, some 156,000 American, British, and Canadian forces landed on five beaches along the Normandy coast of France in the largest amphibious military assaults in history. Over 4,000 Allied troops were confirmed dead and thousands were wounded or missing. German losses are estimated to have been between 4,000 and 9,000 soldiers.

By late-August 1944, all of northern France was liberated. By the following spring, Allied forces had defeated Nazi Germany.