Since it is well known that in Lebanon nothing is unbelievable, the overnight creation of a five-mile-wide ministate by Christian militias was at first greeted nonchalantly in Israel.

The latest escapade of Maj. Saad Haddad, the irascible leader of the rightist militias in southern Lebanon, stimulated Israeli wags to devise names for the new northern neighbor - "Enclavia" became a favorite - and wonder whether it would join the Arab League or the World Zionist Organization.

But the humor has begun to wear thin. The terrifying sounds of artillery duels echo across the northern border every day as the Israeli-supported militias exchange gunfire with both the United Nations peacekeeping force and Palestine Liberation Organization units and the Palestinians and Israeli Army trade volleys on the side.

If that failed to bring home the message, the front page newspaper photographs today should have succeeded. They show Palestinian guerrillas holding Soviet-made Kalashnikov assault rifles in one hand and waving with the other to convoys of Lebanses Army troops moving south toward Haddad's narrow fiefdome.

For the first time in nearly a year, Israel's long-dreamed-for cordon sanitaire in southern Lebanon - the object of the controversial invasion in March, 1978 - is seriously endangered.

With the Lebanese government in Beirut vowing to break up Haddad's splinter army and arrest the Phalangist leader on charges of "high treason," Israel is facing the uncomfortable prospect of having what it perceives as a hostile, Syrian-dominated Lebanese Army as the only buffer between it border and the network of Palestinian bases spread throughout southern Lebanon.

In the view of Israeli Defense Ministry officials, the proverbial fox could soon be guarding the chicken coop.

With cross-border terrorist raids into Israel increasing alongside Arab anger over the Egyptian-Israeli peact treaty, southern Lebanon has once again become a national issue here, eclipsing in importance even the impending normalization of relations with Cairo.

Although Prime Minister Menachem Begin frequently dwells on Israel's role in saving Lebanon's Christian minority from "physical annihailation" at the hands of Syrian troops, Haddad's real use to Israel has never been a myster.

It was clear last June 13 when the Israeli Army ceremoniously handed over control of southern Lebanon to Haddad's 700-man army, and deliverately snubbed the United Nationsforce in Lebanon (UNIFIL), which had a peacekeeping mandate from the U.N. Security Council.

Haddad's force has grown to at least 1,500 men.

Three months earlier. Israel had invaded southern Lebanon in retaliation for a terrorist attack on a tourist bus near Tel Aviv.

The 5,000 U.N. soldiers in souther Labanon never have had the confidence of Israeli defense officials, who say the peacekeeping troops look the other way when terrorist teams head for Israel at night, and that the U.N. forces openly fraternize with the guerrillas to the point of eating meals at their houses.

The recent arrest of a Senegalese U.N. soldier caught smuggling explosives into Israel in the spare tire of his jeep did little to diminish Israel's misgivings about the effectiveness of UNIFIL.

So there was little wonder at Israel' increasing reliance on Haddad, who had seized control of a garrison of Lebanese troops in 1976 when the Syrian Army moved into Lebanon to end the civil war betwen the rightist Christians and the predominantly leftist Palestinians and Moslems.

For the past 10 months, Haddad has controlled a five-mile-wide belt stretching 62 miles across Lebanon from the Mediterranean to the foothills of Mount Hermon, and, until recently, terrorist raids across Israel's borded had ceased.

For its part, Israel - without ever publicily admitting it - has gratefully provided Haddad and his army with money, weapons, munitions and stores.

But, in June, 1978, as now, two major problems plagued the Israeli-Christian arrangement.

The first was the danger of having the tail wag the dog, because of Haddadhs track record of precipitous - and sometimes unrestrained - action. The second was that the government in Beirut, if persistently prodded by Syria, would not long tolerate an independent private army operating freely in the south.

Israeli officials have been reticent about the Lebanese situation, partly for fear of appearing too close to Haddad and partly out of a reluctance to further alienate the United Nations and the member countries whose peacekeeping troops have tasted the might of Haddad's 130-mm guns.

A high-ranking government source said yesterday in a briefing, however, that he does not believe Haddad actually intends to set up his "Free State of Lebanon" in the south. Yet his troops bombarded U.N. Nagoura headquarters this week.

Israeli officials said the Lebanese Army battalion in the south has only 500 men, three armored personnel carriers and four mortars, and in a direct clash would be no match for Haddad's army. Nevertheless, it could be strengthened anytime and pose a real threat to the Christian militias.

The United Nations, under the pressure of its Security Council mandate to establish a stable Lebanese force in the south, no doubt will continue to encorage Beirut to send more troops into the vicinity of Haddad's shaky enclave.

For the moment, it appears that all Israel can do is hope that the regular Lebanese Army units move no farther south than the U.N. buffer - which is north of the Christian enclave - and leave Haddad alone to control Israelhs precarious security belt. CAPTION: Map, no Caption, By Richard Furno - The Washington Post