Britain entered World War I in 1914 to honor a treaty that promised to defend Belgium and to fulfill an alliance with France and Russia. The Germans attacked Liege, Belgium, in the first battle of World War I, and then marched on to attack France.
I'm assuming by "war" you mean World War I
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Their ongoing hostility erupted into civil war in 31 B.C.E as the Roman Senate, at Octavian's direction, declared war on Cleopatra and proclaimed Antony a traitor. Later that year, Antony was defeated by Octavian's forces at the Battle of Actium.
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