'MUNCHING a porcupine, quill by quill" is how Winston Churchill once described the hellish combat in World War II's China-Burma-India theater. The quip is apropos of the War on Terror as well. And Syria seems to be our newest quill.

Ignoring complaints and warnings from most of the rest of the world, Syria continues to sponsor terrorism, pursue weapons of mass destruction (WMD) and allow foreign fighters and terrorists to seep into Iraq. This behavior must meet serious consequences, and soon.

Damascus provides safe haven to such international killers as Hezbollah, Hamas and Palestine Islamic Jihad, which it considers legitimate resistance groups. The Bush administration says that despite promises to close the spigot, militants and Syrian-based terrorists continue to flow unimpeded across the porous Syrian border into Iraq, resulting in the deaths of American and coalition soldiers, Iraqis and international civil servants in recent weeks. (The CIA has identified Syria, Iran and Saudi Arabia as the leading supporters of the insurgency.)

Estimates of the numbers of foreign jihadists in Iraq stretch into the thousands. Indeed, Syrians are so common among captured militants that the coalition military commanders collectively call the foreign radicals: "The Syrians."

Damascus opposed the war in Iraq (this time) and its foreign minister, Farouk al-Sharaa, has made it quite clear: "Syria's interest is to see the invaders defeated in Iraq."

On the mass-destruction front, Syria is most notably developing bugs (biological weapons) and gas (chemical weapons.) It has one of the most advanced chemical-weapons capabilities in the Arab world and is developing an offensive biowar program as well. North Korea and Iran have been helping Syria develop its ballistic missiles, too; they can already reach Israel, Turkey, Jordan and Iraq with chemical munitions.

The latest concern is Syria's nuclear R&D program. Though this is supposedly for peaceful purposes (haven't we heard that before somewhere?), Syria's recent cooperative agreement with Russia on civil nuclear power could help Damascus obtain nukes.

The U.S. government is also troubled by persistent (though unconfirmed) reports that Saddam may have spirited his WMD program into Syria before the war. The Iraqi material could boost Syria's own programs - or be passed along (with the middleman's "fingerprints" carefully wiped off) to its terrorist acolytes for use in Iraq, Israel or elsewhere.

If all that weren't enough, Syria also continues to support Palestinian terrorist groups responsible for recent suicide bombings in Israel. It's no Fantasy Island at home, either, with more than 13 different intelligence and security services providing backbone to the Ba'athist regime.

Though Syria's not yet in the same disrepute as Iran, its leaders seems to be racing the mullahs to succeed Saddam as the Middle East's most vile regime.

Damascus has to pay a price for this behavior:

* Until Syria cleans up its international deportment, it should be shut out of the Israeli-Palestinian Road Map negotiations (London wants Damascus included), and denied any opening on an Israel-Syria track.

This is a real lever: Damascus (which remembers when Syria included what's now Lebanon, Jordan and Israel) is obsessed with the return of the strategic Golan Heights. Israel has had the heights since the 1967 Arab-Israeli War; Syria can't get them back without a peace treaty with the Jewish state.

* Though Syria is already heavily sanctioned by the United States, economic pressure could be multilateralized to include the European Union. Germany, Italy and France are major trading partners. Syria is already experiencing 20 percent unemployment among its young people. Broader economic sanctions could pose a threat to the internal stability of the Syrian police state and hinder its WMD programs.

Syria's 37-year-old ruler Bashar Assad is facing arguably the most critical decisions of his young presidency. He can side with good - or evil. The choices he makes will prove fateful for Syria's 17 million people.

To ensure our national security, the United States must succeed in Iraq, win the War on Terror and prevent the proliferation of WMD. Syria is now a roadblock on all three fronts; we need to clear the way.

Peter Brookes is a senior fellow for National Security Affairs with the Heritage Foundation.