There had been a movement since the 1890s to give dispersed Jews a homeland; the British, who controlled Palestine, were allowing a certain quota of Jews to immigrate each year. After World War II, and partially in response to the Holocaust, the UN, the British, and the Americans set in motion a series of events (positive and negative) that led to Israel’s independence in 1948.
This made the people who had been inhabiting the territory the British evacuated annoyed—in their eyes, the British came in, took over their land, handed it over to a bunch of immigrants, and left. This explains a lot of the origin of the Israeli-Palestinian conflict, and the differences in religion only make it worse.
Further, Israel is a small country, both geographically and population-wise, but it has a lot of strong allies, and so it tends to throw its weight around.
Finally, there’s a long tradition of European anti-Semitism, and there’s no doubt a hefty dose of that here too.