Short answer: Mussolini invaded Abyssinia.

The brutality of the Italian campaign and its lack of justification horrified the League of Nations. Britain was appalled, France had given the green light to Mussolini but had to comply with League of Nations sanctions. The Stresa front was broken, Italy was suddenly on its own and Hitler was right there waiting with a metaphorical box of chocolates.

Source:

Ian Kershaw, Hitler 1889-1936: Hubris, 1998