A one house legislature was proposed.
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The direct cause of WWI was the assassination of Archduke Franz Ferdinand at Sarajevo on 28 June 1914. However historians feel that a number of factors contributed to the rivalry between the Great powers that allowed war on such a wide-scale to break out. I'm not sure who the assassin was.
The fears of the Anti-Federalists arose because the new federal government would have supremacy of the states when it came to legislation--making the Anti-Federalists believe that it would become a tyrannical body. The Constitution tried to put these fears to rest by implementing a system of "checks and balances", wherein no single federal body could become too powerful.