LEBANON is unforgiving terrain to outsiders who would direct its destiny. Israel invaded in 1982 and felt lucky to get out in three years, mission unaccomplished. Syria went in in 1975 and is still stuck, also mission unaccomplished. The Syrians have old ambitions of empire in Lebanon, and President Hafez Assad apparently believes it is a fit place to demonstrate leadership prowess.

Last February, he got in deeper, sending 7,000 troops into Moslem West Beirut, one of the most lawless and anarchic sections of Lebanon. Now he has dramatically upped the ante, sending troops into West Beirut's heretofore inviolate southern suburbs. He did this after his policy was directly challenged 1) by somebody's assassination of a favored Lebanese politician, Prime Minister Rashid Karami, and 2) by somebody's kidnapping of a favored American journalist, Charles Glass. The southern suburbs are a sprawling, slummy, mostly Shiite area, where live the Iranian revolutionary guards, who are now believed to hold up to 25 Western hostages.

Mr. Assad brought the Iranian guards into Lebanon in 1982 to fight the Israelis. They have since taken on a political will of their own. They are not only ungrateful and unruly clients but genuine terrorists. Mr. Assad finds it convenient to come forward now as an opponent of terrorism. It is one of the ways in which he is trying to return to the good graces of the Western world, which reacted strongly against his regime's terrorist activities last year. The Israelis are pleased to see Syria squeezing, for its own reasons, the groups that do the most damage to the lingering Israeli presence in southern Lebanon.

Europe and the United States are uncomfortable condoning an expanded Syrian mission in Lebanon, a country whose independence they profess to uphold. But they see that Lebanon's own government cannot police the country, and they quietly hope President Assad will be able to extract Western hostages -- without getting them killed. Absorbed as they are by their own Lebanese preoccupations, they have an obligation to remind Damascus that its proper mission is not to smother Lebanon but to help restore enough local stability to enable Syrian forces to start getting out