Your question could have multiple answers, depending on context and time frame. Israel has used its military, roadblocks and checkpoints, a blockade of the Gaza Strip, and even has developed nuclear weapons in order to protect their national security and territory.
But perhaps a specific thing you might have in mind with your question would be the construction of the West Bank security barrier. The barrier uses a combination of security fencing and concrete walls to divide Israeli territory from lands in which Palestinians reside. Construction of the barrier began in 2000 and has continued in spite of international objections and internal debate in Israel over its legitimacy. By 2012, 273 miles of the barrier had been completed (about 60% of its planned extent). Current Israeli Prime Minister Benjamin Netanyahu, in February, 2016, announced plans to extend the security barrier project to surround Israel at all borders.
