The iron law of oligarchy is a political theory, first developed by the German sociologist Robert Michels in his 1911 book, Political Parties. It states that all forms of organization, regardless of how democratic or autocratic they may be at the start, will eventually and inevitably develop into oligarchies.
Robert Michels was disturbed to find that, paradoxically, the socialist parties of Europe, despite their democratic ideology and provisions for mass participation, seemed to be dominated by their leaders, just as the traditional conservative parties.
Studying political parties, he concluded that the problem lay in the very nature of organizations. Modern democracy allowed the formation of organizations such as political parties, but as such organizations grew in complexity, they paradoxically became less and less democratic. Michels formulated the "Iron Law of Oligarchy": "Who says organization, says oligarchy."
Now, Michels ended up working for Mussolini, so one wonders if his Iron Law was something he demoaned or exploited. In any case, read more of his thoughts here.