Final answer:
The assassination of the heir to the Austria-Hungary throne sparked the declarations of war between competing European powers in 1914.
Step-by-step explanation:
The event that sparked the declarations of war between the competing European powers in the summer of 1914 was the assassination of the heir to the Austria-Hungary throne.
On June 28, 1914, Archduke Franz Ferdinand of Austria-Hungary was assassinated by a Serbian nationalist. This event, known as the assassination of Sarajevo, led Austria-Hungary to issue an ultimatum to Serbia, resulting in a chain reaction of declarations of war between various European powers.
Germany supported Austria-Hungary, whilst Russia supported Serbia. This eventually led to the outbreak of World War I.
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