From the start of Lebanon's decade of violence in 1975, hostage-taking has been a favorite tactic. But the plight of 40 Americans hijacked aboard a TWA airliner by Shiite Moslem gunmen far from Beirut constitutes a new chapter in Lebanon's headlong descent into anarchy and disorder.

Abruptly, the primitive "us or them" philosophy of blindly taking hostages at street corners to exact concessions from local adversaries has been extended to international politics by detaining civilians with no direct links to local or regional conflicts.

Nabih Berri, the leader of Amal, the largest Shiite militia, has backed the more radical Shiite hijackers' demands that the American hostages be exchanged for more than 700 Shiite Moslems and other Arabs removed from southern Lebanon and incarcerated south of the Israeli port of Haifa when the Israeli Army went home.

After so many years of violence, Berri and many other like-minded Lebanese seem to find it normal that his private militia -- and not the withered Lebanese state -- should offer to exchange two groups of prisoners that he feels have been illegally detained.

Such is the legacy of a decade during which Lebanese warlords and their militias have displaced the Lebanese state and contracted alliances with foreign powers ranging from France and the United States to Syria and Israel.

That sense of anarchy has been increased by Amal's inability to disarm Palestinian guerrillas in the Beirut camps, despite Syria's apparent blessing; suggestions that the hostages are part of a power struggle between Berri and more extremist Shiite leaders, and the recurrent blind violence of car bombs.

Syria, which for 15 months has exercised the greatest foreign influence in Lebanon, reportedly is displeased with the hijacking and Berri's handling of the hostage issue.

But President Hafez Assad of Syria is renowned for his special sense of timing, which many westerners have found infuriatingly slow. Far from preparing for an imminent military intervention to set the Lebanese house in order, Assad seems content to use the Shiites to drive the Israelis from southern Lebanon before moving on the political front in Beirut.

Lebanese President Amin Gemayel has played no visible role in this or any other recent major crisis. All but disowned by his own Maronite Christian community and distrusted by Lebanon's Moslems, Gemayel has been powerless and isolated since his Christian-led Army was swept out of predominantly Moslem west Beirut in early 1984 by Amal and Walid Jumblatt's Druze militia.

The right-wing Christian militia, the Lebanese Forces, has remained largely aloof from developments in west Beirut, witnessing, and sometimes indirectly encouraging, the radicalization there that serves to buttress its claims to be a bulwark against Moslem fanatics.

Amal and the Druze militia, soon after their takeover, trounced the Mourabitoun, the militia of the Sunni Moslem community that once shared political power with the Maronites. It now seems a likely loser in any future reforms of Lebanon's creaky political institutions. The anti-Mourabitoun operation was repeated last April.

Only yesterday, Sunni Prime Minister Rashid Karami staged a sit-in at a Sunni mosque as Beirut's Sunnis held a general strike to protest Shiite hegemony and their own diminution in what was once their undisputed part of the capital.

Despite misgivings about past Palestinian excesses, Beirut's Sunnis looked askance at Amal's month-long siege of fellow Sunnis in the Palestinian refugee camps.

And other representatives of the mainstream Moslem sect -- from the conservative Persian Gulf states to radical Libya -- openly criticized not only Amal, but also its Syrian backers in that exercise.

As head of the small Druze community, normally pro-Syrian Jumblatt openly aided the Palestinians for fear that triumphant Shiites would diminish his hard-won role as the Maronites' principal interlocutor and eventually swamp his relatively sparsely populated Chouf Mountain heartland.

But back in February 1984, the Druze joined the Shiites in taking over west Beirut from the Christian-led Army. That pivotal event triggered the American decision to remove the Marines from Beirut and the other western members of the multinational force soon followed suit.

It also forced Gemayel to travel to Damascus and renounce a U.S.-brokered Israeli-Lebanese troop withdrawal agreement.

Infuriated by the U.S. 6th Fleet's shelling of the Beirut area during the 1984 battles and Israel's refusal to evacuate the predominantly Shiite south, Berri's Amal and the more radical, Iranian-backed Hezbollah, or fundamentalist "Party of God," proved effective allies.

Long the hewers of wood and drawers of water for the other Lebanese communities -- when they were not serving in their militias -- the Shiites gained prominence in their conflict with the United States and Israel.

The most recent militia to appear on the scene, their men fought with the fervor of a community convinced that the time had come to play a major role as the largest single religious group in Lebanon.

For the first time in contemporary Arab history, the "national resistance" -- as the Shiites called their fighters -- were seen by the Arab world to be driving the Israelis out of southern Lebanon.

Diplomats say Syria will continue to back all the Shiite militias until the Israeli Army withdraws its support from its Christian surrogates of the South Lebanon Army in Jezzin and evacuates the border strip held since 1976.

That does not necessarily mean Syria approves of the hijacking or of Berri's involvement in the hostage affair. Indeed, Vice President Abdul Halim Khaddam reportedly made clear his disapproval to various Palestinians earlier in the week.

Two days before the hijacking, Assad denounced the kidnaping of diplomats. Addressing Sunni religious leaders, he spoke of the "principle of honor among warriors" and mentioned his frustration in being unable to arrange the release of Americans detained in Lebanon despite a telephone call from President Reagan asking him to do so.

At this stage, analysts say Assad has only limited leverage over Berri and Hezbollah and probably is loathe to dissipate his influence with such an effective force by intervening in the hostage dispute.

Diplomats say Syria would be prepared to enforce a truce along the southern border similar to the one that has prevailed on the Golan Heights since the 1973 war with Israel, if Israel were to get all the way out of Lebanon.

That offer implies that Syria would back Berri's Amal, which has said it is willing to keep the peace along the border, in contrast to Hezbollah, which is sworn to carry the fight into Israel.

Only then would Syria tackle the Lebanese political reforms that might start putting the country back together again, it is argued here.