In the West, student protests such as strikes date to the early days of universities in the Middle Ages, with one of the earliest being the University of Paris strike of 1229, which lasted two years, and University of Oxford strike of 1209. In more recent times, student demonstrations occurred in 19th century Europe, for example in Imperial Russia. In 1930s, some Polish students protested against anti-Semitic ghetto benches legislation. In the second half of the 20th century, significant demonstrations occurred in the late 1960s and early 1970s: the French May 1968 events began as a series of student strikes; Polish political crisis that occurred the same year also saw a major student activism. The largest student strike in American history occurred in May and June 1970, in the aftermath of the American invasion of Cambodia and the killings of student protesters at Kent State University in Ohio. An estimated four million students at more than 450 universities, colleges and high schools participated in what became known as the Student Strike of 1970.