The Israeli government's decision to launch a punishing offensive against Hamas in the Gaza Strip was triggered by developments on the ground, but it came as two political clocks were ticking at home and abroad: an approaching election in Israel and a transition of power in Washington later this month.

For the three leaders guiding the military campaign—Prime Minister Ehud Olmert, Defense Minister Ehud Barak and Foreign Minister Tzipi Livni—security concerns, political considerations and diplomatic calculations were all part of the mix as they charted their course, analysts say.

The assault on Hamas was meticulously planned for months by the Israeli army. But it was only approved by Barak after a six-month cease-fire expired Dec. 19 and militants in Gaza stepped up rocket fire at Israel following the killing of three Hamas gunmen in a border clash.

With public pressure building for a military response to the rocket attacks and an election campaign under way, all members of the decision-making triumvirate — Olmert, Barak and Livni—had strong incentives to act, analysts said.

Primarily, the leaders had to address the renewed rocket challenge by Hamas, which may have calculated that a broad offensive was unlikely to be ordered by a lame-duck prime minister and an outgoing government in the run-up to an election set for Feb. 10.

The decision was to "change the equation," in the words of Livni, and weaken Hamas to the point that it would accept a cease-fire that would halt years of incessant rocket attacks.

"The objective is to create a new balance of deterrence that would enable a stable and enduring cease-fire," said Shlomo Brom of the Institute for National Security Studies at Tel Aviv University.

But other factors may well have contributed to the decision to launch the offensive now.

Olmert, in his last months in office, tainted by a corruption scandal and an inconclusive war against Hezbollah guerrillas in Lebanon in 2006, "didn't want to go down as a complete and utter failure" and wanted to "rescue some of his legacy," said Reuven Hazan, a professor of political science at the Hebrew University of Jerusalem.

Barak and Livni, contending in the elections against Benjamin Netanyahu, the hawkish leader of the Likud party, could ill afford to be seen as passive in the face of the ongoing challenge from Gaza.

"They couldn't run an election campaign from late December to early February and just be restrained, letting Hamas end the truce and target the Israeli civilian population," Hazan said. "This would have handed the election to Netanyahu."

Barak took a high profile from the start of the Gaza offensive, announcing it to the nation in the hours after the airstrikes began—upstaging Olmert—then outlining the operation's goals in parliament. A poll published Thursday in the Haaretz newspaper showed significant gains for Barak's Labor party and a surge in his approval rating after months of dismal showings.

Livni, who leads the Kadima party and has been portrayed by her political rivals as lacking the experience to be prime minister, also stands to gain by looking tough. She, too, has taken a high profile since the offensive began, visiting sites of rocket attacks and defending the offensive in repeated appearances on international television channels.

It also was a diplomatically opportune moment to launch a broad military assault. Israel had been attacked with rockets and could make a strong argument internationally for a tough response. And the Bush administration, strongly supportive of Israel, was not considered likely to stand in the way, especially in the last weeks of its term.

Delaying the offensive until President-elect Barack Obama took office might have presented another set of problems, although Obama is not expected to depart from Washington's shunning of Hamas or its traditional alliance with Israel.

"On the one hand, you have an outgoing administration that has been very supportive of Israel when it comes to the war on terror. With the new administration, which has said it wants to have a different attitude toward solving problems, such as negotiating with Iran, we could have had the U.S. imposing a cease-fire within days," Hazan said.

Other analysts said the approaching transition in Washington did not appear to be a significant factor in the timing of the offensive, and that even if it were launched after Obama took office, his administration likely would have given Israel similar support on the grounds that it was exercising its right to self-defense.

What's more, Brom said, "the purpose of the military campaign is to achieve diplomatic ends, and the Obama administration would probably be more effective in this area."

The Gaza offensive may end by Obama's Jan. 20 inauguration, but its reverberations will still be felt when he takes office, and they could give the Israeli-Palestinian conflict more urgency on his foreign policy agenda.

For the Israeli leaders, the military campaign is a high-stakes gamble. If the army becomes bogged down in a costly ground invasion of Gaza, or if the rocket attacks are not ultimately halted, the strong public support for the offensive could quickly evaporate, and Barak and Livni's political fortunes could sink, much as Olmert's popularity plummeted after the war against Hezbollah, which was seen in Israel as a failure.

"The wars of the 21st Century are not wars that end in decisive victory," Brom said, referring to conflicts with insurgents or guerrilla groups. "Usually the public feels frustrated when the war ends, because its expectations are not fulfilled. The Israeli public wants to see a decisive victory, but there are no decisive victories in these kinds of wars."