Which of these happened FIRST, following the assassination of Archduke Franz Ferdinand on June 28, 1914?

Austria-Hungary declared war on serbia.

Explanation:

The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. ... When Russia began to mobilize due to its alliance with Serbia, Germany declared war on Russia.

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d.

the northwest territory became the most densely populated area in the u.s.