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Italy made an alliance with Great Britain, France, and Russia in a secret Treaty of London on 26 April, 1915.
Italy was offered Italian-populated provinces in the boundaries of Austria-Hungary.
On 23 May, 1915 Italy declared war on Austria-Hungary. They wanted to start offensive across the valley of Isonzo river.
Italian's initial advance was halted because of flooding in Isonzo river and trench warfare(dig-ins) set in by the opposition.
However, they determined to progress and invaded the valley which was famously called as the Battle of Isonzo.
There were a series of 12 battles. Italians achieved nothing in the first four but paid 280,000 soldiers lives. The sixth Battle of Isonzo did win Gorizia town for Italy.
In the twelfth Battle of Isonzo also known as the Battle of Caporetto(October 1917), Germany helped Austria-Hungary to win a definite victory against Italy.
Austria attempted to invade Serbia thrice in 1914, which turned everything a failure by Serbian counterattacks.
Germany wanted close the account with Serbia once and for all. On 6 September 1915, Central powers concluded a treaty with Bulgaria to whom they gave a better offer than the Allies, a territory in Serbia. Bulgaria and Serbia had long standing issues since 1885.
Serbians were attacked by Austro-German forces on the south and Bulgars from the east.
The surprised Allied forces send help through neutral Greece's Macedonian port of Salonika which was known as the Macedonian Front or Salonika Front. But the Allied troops were not able to penetrate into the Serbian border crossing the Bulgarian army.
The Serbian army, meanwhile begun to retreat westward to refuge in the island of Corfu.
In 1916, the Allied forces reverted as Serbians regained their life at Corfu. But Bulgarian army started to invade most of the regions of Greece because of Greek's helping hand to the Allies. Political situation in Greece was confused on whom to support.