President Mohamed Morsi of Egypt took office less than two months ago and has been struggling to steer his country, its economy and its politics on a post-Hosni Mubarak course. Now another steep new challenge is on the Islamist leader’s list, after a border attack in the Sinai Peninsula killed 16 Egyptian soldiers and turned longstanding security concerns into a full-blown crisis.

The attack will test Mr. Morsi’s ability to establish control over the lawless Sinai, and it is also a test of his approach toward Israel. Whatever divides the two countries — and there is plenty — they are tightly bound at the Sinai border. There will never be true stability if Egypt cannot find ways to work with Israel on security issues and to continue honoring their 1979 peace treaty.

The Sinai was largely demilitarized after the treaty. Mr. Mubarak cooperated with Israel on security but neglected the desert region, and many residents feel disenfranchised. Since Mr. Mubarak was toppled in 2011, the Sinai has experienced a growing lawlessness as Bedouin criminals, Palestinian militants from neighboring Gaza and other extremists operated at will. Weapons are plentiful and state institutions few.

According to witnesses and officials, masked gunmen stormed a border post on Sunday night and killed the soldiers as they prepared to break their Ramadan fast. The gunmen then barreled into Israel in a stolen truck and armored vehicle; one vehicle exploded at the border and the other was struck by Israel’s Air Force.

The attackers have not been definitively identified. Mr. Morsi’s Muslim Brotherhood Party, which has often voiced hostility toward Israel but was not speaking for the president, used its Web site to suggest that Israeli intelligence could be to blame. Israel’s ambassador to the United States used Twitter to finger Iran but his government, and many others, see the work of Al Qaeda-inspired militants with ties to Palestinians in Gaza. Egypt’s military, which still holds many levers of power, called the attackers “infidels.” Mr. Morsi also ignored the anti-Israel line. He said, “There’s no room to appease this treachery, this aggression and this criminality,” and he promised that security forces would extend “full control” over the Sinai.

There are some early positive signs. According to The Times, an Israeli general and an Egyptian general met near the border to discuss an investigation into the incident. Israel handed over to Egypt the armored car and the bodies of those killed. Egypt closed the border crossing with Gaza and began sealing off smuggling tunnels.

To substantially beef up military assets in the region, the government may have to modify its treaty with Israel. Meanwhile, Mr. Morsi has been confronted with the anger of Egyptians who consider him and the Brotherhood sympathetic to the soldiers’ killers, in part because he has made overtures to Gaza’s Hamas leaders. Protesters forced Mr. Morsi to drop plans to attend funerals for the soldiers on Tuesday.

It is too soon to say whether the Egyptian government and the military, still focused on a difficult democratic transition, will make the volatile Sinai a priority. It is in the national interest, something they are still struggling to understand.