Final answer:
Germany and Great Britain competed over the size of their navies in an arms race during the 1900s.
Step-by-step explanation:
The two countries that began to compete over the size of their navies in an arms race in the 1900s were Germany and Great Britain.
Germany, under the leadership of Kaiser Wilhelm II, aimed to challenge Great Britain's naval supremacy in an attempt to become a global power. This led to an escalation of naval building and increased tensions between the two nations.
One significant event during this arms race was the construction of the German battleship Kaiser Wilhelm der Grosse, which posed a direct challenge to Great Britain's naval dominance.
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