In 1978, Israel and Egypt agreed that Sinai would be a demilitarized zone. Upon reaching the agreement, Israel pulled out of Sinai, Egypt was given sovereignty over it, and U.N. peacekeepers were put in place to monitor the situation.
While Hosni Mubarak was president of Egypt, this agreement worked because his government was not hostile to Israel. But following the election of "a Muslim Brotherhood government in Egypt," the agreement was immediately strained, and "communication and coordination" between the two countries rapidly declined.
Now, Egypt is ignoring the 1978 agreement by placing military forces in Sinai, and Israel has responded by issuing an official objection to the military buildup -- a buildup which certainly could spell trouble for Israel if hostilities break out.
According to the World Jewish Daily, the Israelis had their objection conveyed to Egypt via the White House in an effort to demonstrate how serious the situation has become.