After the Camp David accords, the Sinai has been largely a demilitarized zone, since Israel has been fearful of having too many Egyptian troops close to its border; UN troops kept the sides apart for decades. However, that has left a vacuum where Gaza gun runners and local goons can operate without fearing a major armed backlash from the authorities, especially after the Arab Spring ditched Mubarak last year.
That has changed; with Israel's blessing, the Egyptian army is moving back in.
Hundreds of troops in armored cars drove out of the town to hunt Islamist militants blamed for killing 16 Egyptian border guards on Sunday, the biggest spike in violence which has been growing steadily since last year's overthrow of Hosni Mubarak.
The gunfire in al-Arish, the nerve centre of the government's otherwise shaky control of the North Sinai region, showed how difficult it will be for Egypt to impose order. It followed attacks on checkpoints in the town on Wednesday.
Israel has welcomed Egypt's offensive while continuing to express worries about the deteriorating situation in Sinai, home to anti-Israel militants, Bedouin tribes angered by neglect by Cairo, gun-runners, drug smugglers and al Qaeda sympathizers.
Israeli Defense Minister Ehud Barak said Egypt was acting "to an extent and with a determination that I cannot previously recall".
It also puts new President Mursi in an awkward spot; Hamas is a Muslim Brotherhood offshoot, so he'd be going against some spiritual allies in cracking down on the smuggling of military goodies into Gaza. However, if Mursi is going to have any credibility with the west as an Islamic democrat, keeping Hamas at a distance and Israel as a peaceful neighbor would require such a crack-down.
That bode watching, for if Mursi can pull that off, there might be hopes for some form of housebroken Islamic democracy; the "Christian Democrats" of Europe come to mind as a bloc that is religious but not authoritarian. It would be a bit of a feat to see an Islamic Democrat when Islamic doctrine leans toward a church-state fusion.