After the of Archduke Ferdinand, many countries joined WWI because of that had been made. These countries had a mutual agreement to defend one another against attack.

assassination; the law

Explanation:

Archduke Ferdinand was assassinated along with his wife Sophie, and his death triggered Austria and Hungary to fight. Alliances were made until WWI became known. The Treaty of Versailles ended it nearly five years later. Some people were forced into the militia so a law was created (can't remember which country) while others went willingly.

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i think it's a)sparta