Great Britain is a Island Nation, and without control of the water, the British cannot import materials, food, and other necessities it needs to survive, feed it's population, and build weaponary. So the control of the ocean is essential for their very survival
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These alliances that took place before World War 1 in a way started the war. For an example, Germany was in an alliance with Hungary and when they declared war against Serbia, Germany had to fight with them, and so on, finally creating World War 1. Within weeks, Germany declared war with other countries that declared war with others, creating a "domino effect" which just means that most countries just started fighting with one another, either because they declared war with them or because their allies did.
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