Answer:
Germany
Explanation:
The war was started by the leaders of Germany and Austria-Hungary. Vienna seized the opportunity presented by the assassination of the archduke to attempt to destroy its Balkan rival Serbia.
In the Age of New Imperialism that began in the 1870s, European states established vast empires mainly in Africa, but also in Asia and the Middle East. - https://www.tamaqua.k12.pa.us/cms/lib07/PA01000119/Centricity/Domain/119/TheAgeofImperialism.pdf
Austria used it as a flimsy excuse to start a war with Serbia, which it had been wanting to do anyway.
In June 1812, the United States declared war against Great Britain in reaction to three issues: the British economic blockade of France, the induction of thousands of neutral American seamen into the British Royal Navy against their will, and the British support of hostile Indian tribes along the Great Lakes frontier.
