<P> The German occupation of Norway in April 1940, the rapid conquest of the Low Countries and France in May and June and the Italian entry into the war on the Axis side in June transformed the war at sea in general and the Atlantic campaign in particular in three main ways: </P> <Ul> <Li> Britain lost its biggest ally . In 1940, the French Navy was the fourth largest in the world . Only a handful of French ships joined the Free French Forces and fought against Germany, though these were later joined by a few Canadian - built corvettes . With the French fleet removed from the campaign, the Royal Navy was stretched even further . Italy's declaration of war meant that Britain also had to reinforce the Mediterranean Fleet and establish a new group at Gibraltar, known as Force H, to replace the French fleet in the Western Mediterranean . </Li> <Li> The U-boats gained direct access to the Atlantic . Since the English Channel was relatively shallow, and was partially blocked with minefields by mid-1940, U-boats were ordered not to negotiate it and instead travel around the British Isles to reach the most profitable hunting grounds . The German bases in France, at Brest, Lorient, and La Pallice (near La Rochelle), were about 450 miles (720 km) closer to the Atlantic than the bases on the North Sea . This greatly improved the situation for U-boats in the Atlantic, enabling them to attack convoys further west and letting them spend longer time on patrol, doubling the effective size of the U-boat force . The Germans later built huge fortified concrete submarine pens for the U-boats in the French Atlantic bases, which were impervious to Allied bombing until mid-1944 when the Tallboy bomb became available . From early July, U-boats returned to the new French bases when they had completed their Atlantic patrols . </Li> <Li> British destroyers were diverted from the Atlantic . The Norwegian Campaign and the German invasion of the Low Countries and France imposed a heavy strain on the Royal Navy's destroyer flotillas . Many older destroyers were withdrawn from convoy routes to support the Norwegian campaign in April and May and then diverted to the English Channel to support the withdrawal from Dunkirk . By the summer of 1940, Britain faced a serious threat of invasion . Many destroyers were held in the Channel, ready to repel a German invasion . They suffered heavily under air attack by the Luftwaffe's Fliegerführer Atlantik . Seven destroyers were lost in the Norwegian campaign, another six in the Battle of Dunkirk and a further 10 in the Channel and North Sea between May and July, many to air attack because they lacked an adequate anti-aircraft armament . Dozens of others were damaged . </Li> </Ul> <Li> Britain lost its biggest ally . In 1940, the French Navy was the fourth largest in the world . Only a handful of French ships joined the Free French Forces and fought against Germany, though these were later joined by a few Canadian - built corvettes . With the French fleet removed from the campaign, the Royal Navy was stretched even further . Italy's declaration of war meant that Britain also had to reinforce the Mediterranean Fleet and establish a new group at Gibraltar, known as Force H, to replace the French fleet in the Western Mediterranean . </Li> <Li> The U-boats gained direct access to the Atlantic . Since the English Channel was relatively shallow, and was partially blocked with minefields by mid-1940, U-boats were ordered not to negotiate it and instead travel around the British Isles to reach the most profitable hunting grounds . The German bases in France, at Brest, Lorient, and La Pallice (near La Rochelle), were about 450 miles (720 km) closer to the Atlantic than the bases on the North Sea . This greatly improved the situation for U-boats in the Atlantic, enabling them to attack convoys further west and letting them spend longer time on patrol, doubling the effective size of the U-boat force . The Germans later built huge fortified concrete submarine pens for the U-boats in the French Atlantic bases, which were impervious to Allied bombing until mid-1944 when the Tallboy bomb became available . From early July, U-boats returned to the new French bases when they had completed their Atlantic patrols . </Li>

How many battles did the allies win in ww2