What made wwi the first “modern” war?

WW1 was considered the first 'modern' war, because of the new inventions and technologies that they had, including the usage of planes, the invention of tanks, the discovery of gas (such as mustard gas), and others

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Britain and its empire stood alone against hitler and mussolini. in june 1941, hitler broke the non-aggression agreement with stalin and germany invaded the soviet union. in december, japan attacked the us and britain. the main lines of world war ii had formed.