Jerusalem — The bomb that shattered the United States Embassy in Beirut - causing dozens of fatalities - bluntly drove home two contradictory messages to the US government:
* The urgency of a speedy end to negotiations over Lebanon's future.
* And the unlikelihood that the United States can arrange this.
The blast came at a time when both Israel and Lebanon were reporting progress in negotiations for withdrawal of Israeli troops from Lebanon.
Such an agreement has top priority for the US in the wake of the blow to US-Mideast diplomacy dealt by Jordan's decision not to enter wider peace talks based on President Reagan's peace proposal.
The explosion highlighted the weakness of the Lebanese government. It was a stunning reminder that even should Lebanon and Israel reach an agreement, the acquiescence of the Syrians - and their Soviet ally - will be needed before all foreign troops leave Lebanon and US diplomacy can chalk up a success.
Responsibility for the blast was claimed by a pro-Iranian group called Jihad al-Islami (Muslim Holy War), which Lebanese police say is linked to a radical faction of the Lebanese Shiite Muslim militia, Amal. Subsequently, other groups claimed responsibility.
The Lebanese Shiites - an Islamic minority sect - are co-religionists of Iran's Shiites and are linked to the minority Alawite sect of Syria's President Hafez al-Assad. The pro-Khomeini Amal fighters are based in Lebanon's Bekaa Valley, where thousands of Syrian troops as well as contingents of Iranian Revolutionary Guards are also based.
Jihad al-Islami has claimed several previous attacks on troops of the multinational force stationed in Beirut. Whichever group actually committed the act, the ability to hit what should have been the best-protected embassy in Beirut underlines the continued weakness of the Lebanese Army and security forces, fragmented by years of civil war, and now being retrained with US help.
With the resurgence of Lebanese terrorist factions, French, Italian, US, and British soldiers become increasingly vulnerable. Yet their presence will be of critical importance in helping the Lebanese Army maintain order if progress is made toward withdrawal of foreign troops from Lebanon.
President Reagan has stressed the blast will not alter the US commitment to a solution in Lebanon, but signs of congressional dismay elicited by the disaster hint at future domestic pressures to cut short any long-term US troop presence. At the broader level, the blast provides a vicious preview of the problems the US faces in reconciling the opposing positions of the foreign powers involved in Lebanon.
Senior Israeli Cabinet ministers insisted Tuesday that the explosion proves terrorism still dominates Lebanon. They said it justifies Israeli insistence on firm guarantees for the security of its northen border.
Although there is no evidence of Syrian involvement in the blast, the Shiite link with the Syrian-controlled Bekaa points up the Syrian role in any Lebanese settlement. For even if Israel and Lebanon sign an agreement, no one is certain the Syrians will accept it.
US special envoy Philip C. Habib has repeatedly assured the Israelis the Syrians are ready to pull back. But Mr. Habib has not visited Damascus this year. Israeli officials have expressed skepticism that Syria's 40,000 soldiers in Lebanon, or the 7,000 to 10,000 Palestine Liberation Organization (PLO) troops still in north and east Lebanon, will leave.
Some Israeli military analysts say the presence of Israeli troops only 21 miles from Damascus and the danger of another Israeli-Syrian war will persuade Assad to pull back.
Other analysts point to Syria's renewed military self-confidence, bolstered by a larger Soviet presence in Syria, installation of Soviet-made SAM-5 missiles there, and Soviet resupply of the Syrian military. Moreover, they note, Syrian deployment in the Bekaa is easier to maintain than its former massive troop presence in Beirut and south Lebanon, where its soldiers were intensely unpopular.
Nor is Saudi Arabia - which failed to pressure the PLO to give the green light to Jordan's King Hussein to enter peace talks - seen as likely to exert irresistible pressure on Syria to pull out.
Intensely aware of the Syrian wild card, President Reagan last week sent a message to President Assad reaffirming the US position that UN Resolution 242, the basis of all US peace efforts in the Mideast, requires an Israeli withdrawal from at least part of the Israeli-occupied Golan Heights. But Israel extended its law to the Golan Heights in 1981 and has said it will never return them.
Mere words are unlikely to win Syrian troop withdrawals, let alone acceptance of any US attempt to revive the Reagan plan. Nor are the Soviets likely to acquiesce in a Mideast Pax Americana without some role in peace negotiations there.
The US is likely to find itself squeezed between the need to woo the Arab side by pressing Israel and the need to avoid alienating Israel from the bargaining table. Israel has made clear it will keep some troops in Lebanon if Syria does not withdraw. The bomb in the US Embassy warns of dangerous negotiating terrain ahead.