These competing interests and rivalries came to
a head with disastrous consequences in late July and early August
1914. One week after Austria-Hungary declared war on Serbia, five
European empires were at war. Germany, Austria's closest ally, quickly
declared war on both Russia and its long-time ally France.
On 4 August, the Germans launched their attack
on France by invading Belgium, thus breaking the 1839 Treaty
of London, which guaranteed Belgian neutrality. Later on the
same day, Britain - one of the original guarantors of Belgian neutrality
and the ally of both Russia and France - declared war on Germany.
The European war that so many had predicted was now a reality.