WHEN YITZCHAK, a young Israeli soldier in the army reserves, returned this June from a backpacking weekend in the Galillee mountains; he learned that the captain of his battalion had called several times looking for him. Nine years earlier, his brother had responded to a similar call by rushing immediately to join the embattled Israeli Army to fight in the sudden Yom Kippur war. This time, Yitzchak (not his real name) skipped out on his army duty. The reason, he says: his part-time job and college homework somehow seemed more pressing.

Israeli citizens, and even once-fiercely loyal soldiers like Yitzchak, are losing some of their legendary fervor for national goals. Many, like the young Jewish soldier, have misgivings about what they see as the unduly militaristic policies of Prime Minister Menachem Begin, particularly the recently-concluded war in Lebanon against the Palestine Liberation Organization.

The nation's deep-seated doubts about its most recent foreign venture, evident to almost any visitor to Israel this summer, are only the most obvious reason why Begin has ceased to be an asset to Israel. He's won some territory and eradicated some foes, but his strident rhetoric, his refusal to accept compromise peace plans, and the exhausted state in which his military forays have left the nation are all holding Israel back. With an opposition Labor Party that seems increasingly realistic waiting in the wings. Begin's time seems to have come and gone.

The Lebanon conflict was stamped as "Begin's war" by some of his fellow countrymen, and Israel was tired of it before it even began. After 34 years of fighting, through four major wars, the nation has reached a saturation point. Its economy is over-wrought (inflation is 130 percent), its founding idealism has diminished, and it feels the time has come to peacefully reap the fruits of nationhood.

The hundreds of Israeli kibbutzim, its cooperative agriculture communities, were once feverishly intent on expanding the nation's wealth through hard work. Today most are building swimming pools or tennis courts--indicating the nation's growing eagerness to consolidate and relax, not expand.

Israelis are also tired of the hard-liners' justifications for conflict. They don't all buy Begin's assertion that Israel should expand to its Old Testament boundaries. They see the need for compromise, after so many trying years attempting to increase national boundaries towards the Biblical ones. Many believe Israel shouldn't be expending its energy or damaging its image abroad by trying to defend such perimeter territories as the West Bank and the Gaza Strip.

BEGIN HAS GIVEN Israel a terrible image problem. His harsh, vituperative tone manifested itself repeatedly this summer, often when he sought to defend Israel's repeated violations of the cease-fires to which it had agreed. His intentions may be laudable, but his faux pas are counterproductive.

This summer, for instance, Begin heard that U.S. Sen. Charles Percy (R-Ill.) had declared that Israel had to be brought to its knees. It turned out that Percy had made no such statement, but Begin angrily responded nevertheless: "Nobody, nobody is going to bring Israel to her knees...the Jews do not kneel but to God."

Most recently, the prime minister's outright rejection of President Reagan's Mideast initiative exasperated the increasing number of moderate Israelis who want to make compromises for peace. That plan includes full autonomy for the Palestinians and would prohibit future Israeli settlements in the contested West Bank and Gaza Strip, so Begin perceived it as a rejection of his own well-laid plans. Certainly the Reagan proposal has features that may territorially slight Israel. But Begin's swift rejection of the plan wholesale led some Israelis to criticize his "hard-headedness" as detrimental.

Israel's public relations abroad, thanks to its perceived belligerence, have sunk to an all-time low. The nation has, of course, surmounted bad press in the past, but now even some of its most ardent supporters have their doubts. The financial and military backing that Israel needs from other nations could wane, and anti-Semitic terrorists could easily mistake the growing skepticism of Israeli actions with support of their horrible anti-Jewish bombings, like the recent ones in France.

But is there an alternative to Begin? Shimon Peres, the leader of the opposition Labor Party whom Begin has edged out twice in national elections, increasingly seems a voice of realism. He has been pressuring Begin to use the Lebanon victory for a generous peace, and probably gains some public support each time Begin avers his refusal to budge. Unlike Begin, Peres received Reagan's peace plan warmly.

It's always possible, of course, that Begin could seize the moment and show the magnanimity he exhibited in the historic Camp David accords. But the prime minister has given no hint that conciliation's in the works, and if he continues in his stubborn ways, he could eventually find himself dumped by a populace grown increasingly impatient with his intransigence.