Why did Italy abandon its alliance with Germany in WW1 and join the Allied side?

Italy's main issue was its enmity with Austria-Hungary, Germany's main ally. That made Italy the "odd man out" in the so-called Triple Alliance with the other two.

Italy had joined (reluctantly) with Germany out of a fear of France. This occurred when France and Britain concluded an alliance that made Britain responsible for the mutual defense of the English Channel, and freed the French fleet to concentrate in the Mediterranean, possibly against Italy.talian fears of France were quite justified. From the end of the fifteenth century to the formation of the Italian state in 1861, France had either invaded Italy or intervened as an ally with a hefty price no less than a dozen times. In each and every instance did the French emerge easily victorious, so it had become apparent by the start of the twentieth century that they were greedily and lustily eyeing Italy once again, though their goals had not clearly been indicated. France was also still smarting from its defeat against Prussia (which would help form the unified German state in 1871), and suffered from a perpetual inferiority complex vis-à-vis Britain with respect to industrial might, as well as to Russia in simple terms of size and manpower. Moreover, the rapid rise of latecomers Japan and the United States as world powers to the point where both could boast of being stronger than France, further contributed to low French esteem among the global leaders.Although it is true Italy felt as "The Odd Man Out" with its alliance with Germany and Austria-Hungary, there are some cultural ties with that of France and Italy that cannot be overlooked. The basic foundation of the "Romance Languages," religion, and history. I also feel that the "Entente" powers were less of a threat to Italy. France also was preoccupied with taking back Alsace Lorraine from Germany, and in fact based its defense of Paris with a counterattack on Germany's right wing, and push into Alsace Lorraine. At this point, Italy was the least of France's concerns. Italy was not a threat to the French but an asset to the cause by opening another point of the Western Front.

Although it is true Italy felt as "The Odd Man Out" with its alliance with Germany and Austria-Hungary, there are some cultural ties with that of France and Italy that cannot be overlooked. The basic foundation of the "Romance Languages," religion, and history. I also feel that the "Entente" powers were less of a threat to Italy. France also was preoccupied with taking back Alsace Lorraine from Germany, and in fact based its defense of Paris with a counterattack on Germany's right wing, and push into Alsace Lorraine. At this point, Italy was the least of France's concerns. Italy was not a threat to the French but an asset to the cause by opening another point of the Western Front.

Firstly it need to be clarified the nature of Italian treaty with Gemany and Austria: this was just and only a "non aggression treaty and mutual protection in case of attack" Germany and Austria have never been attacked, on the opposite they ignited the war. Secondarily needs to be said that Italy was re-united after centurues of occupation from Spain, France, and specially from Austria, with whom Italy fought the hardest independence wars few years before. The "non aggression treaty" was stipulated in order to have some oxigen from the "German invaders" At the ignition of the war, Italy's only certaintly was NON entering in war with its recent enemies. Italywas however a young monarchy with huge internal problems. Italy's main front was on its borders with Austria. Italy had a very hard defeat in Caporetto, and a strong victory in Vittorio Veneto. This front kept millions of Austrians busy on this border with sacrifice of hundreds of thousands of Italians!

Italian fears of France were quite justified. From the end of the fifteenth century to the formation of the Italian state in 1861, France had either invaded Italy or intervened as an ally with a hefty price no less than a dozen times. In each and every instance did the French emerge easily victorious, so it had become apparent by the start of the twentieth century that they were greedily and lustily eyeing Italy once again, though their goals had not clearly been indicated. France was also still smarting from its defeat against Prussia (which would help form the unified German state in 1871), and suffered from a perpetual inferiority complex vis-à-vis Britain with respect to industrial might, as well as to Russia in simple terms of size and manpower. Moreover, the rapid rise of latecomers Japan and the United States as world powers to the point where both could boast of being stronger than France, further contributed to low French esteem among the global leaders. Italy was the one other country deemed to be a major power (albeit debatably), and it also happened to be the only one of these countries that France had a history of soundly beating. Thus, in terms of the pecking order of great powers, France also wanted to make sure (aside from pursuing longstanding interests in Italy) that it was not to be ranked at the very bottom, and seemed intent on pushing that point home to the Italians.

It is easy to explain why Italy didn't join the war: they had little to gain from it, maybe they also didn't feel prepared. Alliances are always theory and a country can refuse to be dragged into a conflict with powers that are much stronger than it.

The question why Italy later still decided to join is more difficult. This website gives the following answer:

In 1915, Italy had signed the secret Treaty of London. In this treaty Britain had offered Italy large sections of territory in the Adriatic Sea region – Tyrol, Dalmatia, Trieste and Istria. Such an offer was too tempting for Italy to refuse. Britain and France wanted Italy to join in on their side so that a new front could open up to the south of the Western Front. The plan was to split still further the Central Powers so that its power on the Western and Eastern Fronts was weakened. The plan was logical. The part Italy had to play in it required military success. This was never forthcoming.