US secretaries of state usually devote their visits to the region shuttling between the Israelis and Palestinians, but as Hillary Clinton began her one-day stopover today, the focus is on two countries needing US mediation: Israel and Egypt.

Instead of filling the diplomatic vacuum that has existed since former leader Hosni Mubarak was ousted in 2011 and bolstering ties between the two neighbors, the recent election of the Muslim Brotherhood's Mohamed Morsi to the presidency has only exacerbated Israeli anxiety about an erosion in the countries' 33-year peace deal. 

Israel, which has so far been unsuccessful at engaging Egypt's new political establishment, is hoping that Mrs. Clinton will shed light on whether Mr. Morsi plans to press for revisions to the longstanding, stable Israel-Egypt treaty, which is unpopular among the Egyptian public. Clinton preceded her Israel visit with a stop in Cairo, where she met with Morsi. 

"Our attempts to reach the Muslim Brotherhood were not very successful," says Eli Shaked, a former Israeli ambassador to Egypt. "We greatly need the US to make sure that the Egyptians aren't making mistakes and thinking about changing the peace agreement with Israel." Mr. Shaked, who has been briefed on diplomacy efforts, says that the Israeli ambassador's efforts to reach out to the new Egyptian administration received no response. 

 Israel's treaty with Egypt includes a limit on Egyptian military deployments in the Sinai peninsula, creating a largely demilitarized buffer between the countries, and Israel fears now that the new president will reopen the issue. In the last year, rising security chaos in the desert peninsula has become a bone of contention between the sides. 

During the presidential campaign, many candidates, Morsi included, argued that the provision regulating Egyptian troop presence in the Sinai needs to be revisited to allow more Egyptian military forces there to boost control and stymie an upsurge of militant attacks from Sinai into Israel (there were two in 2011).

Israel is worried that reopening the treaty could be used as a pretext for anti-Israel politicians in Egypt to erode ties between the two countries and prefers the ad-hoc, mutually agreed-to deployments of Egyptian military reinforcements in Sinai. In a message to the new Egyptian administration, Israel offered its consent to deploying infantry reinforcements, according to Zalzburg. The two countries agreed to similar reinforcements last year, but Israel is unlikely to acquiesce to anything more substantial, such as tanks.

The cross-border attacks are considered merely a tactical thorn for the time being, rather than a real threat to the peace treaty. But they do put pressure on both countries' security establishments to bring an end to such attacks. There's a risk that attacks, like a shooting attack in May which killed one Israeli, could sour the ties and create a real crisis for the peace treaty.

Although Israel fears a bolstered Egyptian presence in Sinai if the security annex is reopened, some see a negotiated reinforcement of the Egyptian military in the desert peninsula as not only an opportunity to cement ties, but to achieve a symbolic recognition of the peace from the  Muslim Brotherhood. 

"Morsi is calling for an updated annex,'' says Ofer Zalzburg, an senior analyst at the International Crisis Group. "This suggests that he is willing to agree to an updated annex and to sign it, and were this to happen, it's overlooked that this will be a significant Islamist endorsement to the legitimacy to the annex and the peace treaty.''

The meat of Clinton's meetings are expected later in the day, in talks with Defense Minister Ehud Barak and Prime Minister Benjamin Netanyahu. In the last three years, Netanyahu and President Obama have openly clashed on how to handle the Palestinians and Iran's nuclear program.

Clinton also met with Palestinian Authority Prime Minister Salam Fayyad, but PA President Mahmoud Abbas was not part of the agenda.

Clinton is just one of three top administration officials to visit Israel this month. National Security Advisor Tom Danilon held a visit last week (which was not made public until after it happened) and Defense Secretary Leon Panetta is expected to visit soon. The focus for those talks is believed to be Iran's nuclear program and the rising chaos in Syria. 

The US has been trying to calm Israeli fears that time is dwindling before Iran builds a nuclear weapon and that it will need to launch a preemptive strike to halt Tehran's weapon development. The impasse in talks between the international community and Iran has raised speculation that Israel is more seriously considering a preemptive strike, which the US hopes to head off.

The two sides also are discussing the potential for the fighting in Syria to spill over the Lebanese and Israeli borders. Israel and the US are particularly concerned that Syria's chemical weapon and missile stockpiles will fall into the hands of Hezbollah. US officials recently warned that some of those weapons had been moved.

Before heading off to her meetings with Mr. Barak and Mr. Netanyahu, Ms. Clinton spoke to reporters of the challenge facing the allies: "It is a time of uncertainty but also of opportunity," she said. "It is at moments like these that friends like us have to think together and act together. We are called to be smart creative and courageous."