Complex sentence: Fascism opposed the international character of mainstream socialism; but in opposing this international character, it sometimes defined itself as a "nationalist" socialism, an alternative to the mainstream socialism that it regarded as its bitter enemy.
Split into simpler sentences: Fascism opposed the international character of mainstream socialism, but sometimes sought to define itself in opposition to it as a form of "nationalist" socialism. In opposing this international character, Fascism sometimes defined itself as a "nationalist" socialism, an alternative to the mainstream socialism that it regarded as its bitter enemy.