Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President George W. Bush from 2001 to 2002, he is the author of six books, including "Comeback: Conservatism That Can Win Again," and is the editor of FrumForum.
Jerusalem (CNN) -- An Israeli observer describes the mood of this country's leadership as it watches the crisis in Egypt: "It varies," he said, "from gloom to doom."
The protesters demanding the end of President Hosni Mubarak's government are not concerned with Israel, much.
They have concerns closer to home: poverty, corruption, political oppression.
But if the protesters succeed in toppling the Mubarak government, Israelis fear that an alternative Egyptian government might prove a much more dangerous neighbor for the Jewish state.
What do Israelis fear? Some scenarios:
1) Right now, Egypt cooperates with Israel in isolating the Hamas regime in Gaza. What if Egypt changes its mind? If Egypt stops policing the border between Gaza and Sinai, Israel will be forced into an ugly choice. Allow Hamas to import weapons or else intrude into Egyptian territory to close the border itself.
2) The Mubarak regime operates an effective if heavy-handed counterterrorism regime inside Egypt. Egypt has produced its share of terrorists: the blind sheikh who tried to blow up New York landmarks, 9/11 ringleader Mohammed Atta. But these terrorists have been compelled by Egypt's efficient police to operate outside Egypt. What if that changed? If the Egyptian police collapse, Cairo -- population 17 million -- suddenly becomes an attractive place for terrorists to hide.
3) Israel enjoys peace on its two longest borders, with Egypt and Jordan. Peace has enabled Israel to reduce defense spending from about one-quarter of GDP in the 1980s to 9% by the end of the 1990s to only 6.7% in 2010. Peace has also freed Israel to concentrate its forces against a shorter list of threats. A hostile turn by Egypt will intensify Israel's security problems and force increases in Israel's defense spending.
4) Israel must worry about the worst-case scenario: a radical fundamentalist regime in Egypt. Such an outcome is repudiated by the urban protesters who talk to reporters. But Egypt is a country of 80 million, half of whom live on $2 a day. A quarter of Egyptians cannot read. What do they think? Who knows? Whom will they follow? Again, who knows?
5) But failing a worst-case scenario, there's an intermediate-range scenario that is also unappealing: A Mubarak replacement decides to appeal to Egyptian nationalism by chilling the relationship with Israel. Not war, exactly, but an end to the security cooperation that has helped normalize Israeli life since 1980.
6) Egypt has been a voice urging Palestinians to cooperate with Israel. Egypt has supported the Palestinian Authority over Hamas, has bolstered Saudi willpower against Iran, has generally shared a vision of a pro-Western orientation for the largest country in the Arab world. That could all change tomorrow, transforming the region's most influential voice for cooperation with the West into the disruptive influence Egypt was in the 1950s and 1960s.
7) A post-Mubarak Egypt could follow the example of Recep Tayyip Erdogan's Turkey: try to bolster its own nationalist credentials by joining the propaganda war against Israel. Mubarak by and large refrains from accusing Israel of imaginary crimes. He does not celebrate Hezbollah. He shuns Iran. He does not allow Egyptian ports to be used by pro-Hamas flotillas. The next government could alter some or all of those policies.
Many Israelis do hope for an optimistic outcome in Egypt: a gradual transition to a more representative parliament, with a Western-oriented president in control of the armed forces and security services. Over the long run, a more democratic and liberal Arab world will enhance Israeli security.
Pending that outcome, many Israelis express bafflement at the enthusiasm their American friends have been expressing for the Egyptian protests. The Israelis seem to say: "We hope the protests are for the best. But you act as if you know -- and you do not know."
The opinions expressed in this commentary are solely those of David Frum.