Tzipi Livni’s decision to join Benjamin Netanyahu’s government is the first bombshell of the post-election, coalition-building season: In a reversal of her pre-Election Day stance, the woman who went on TV to implore the heads of the two leading center-left parties to join her and build an alliance to thwart the unsuitable Netanyahu’s return to the prime ministership is now the first to join the coalition, all but ensuring that Netanyahu will retain the post.

This is the same Livni who passed up the opportunity to lead the nation after her Kadima party leader Ehud Olmert resigned as prime minister more than four years ago because she wouldn’t cut a deal with Shas. Now she’ll be a junior player in a coalition led by her bitter rival, almost certainly with Shas, which is applauding her trailblazing move and preparing to follow suit.

This is also the same Livni who wouldn’t cut a deal with Netanyahu after the last elections in 2009, from a position of relative strength, when her Kadima party (28 seats) was bigger than his Likud (27). She could have been foreign minister, at the very least, with many of her Kadima colleagues in senior ministerial positions, dramatically influencing government policy. But a national unity government with the reviled Netanyahu would be “a coalition that doesn’t allow me to pursue my path, the path … we promised the voters,” she said at the time. Now, from a position of immense weakness, she is the first party leader to break ranks and cut a deal, in exchange for two ministries, lots of assurances, and no means to ensure they are honored.

As the head of a faction with merely six Knesset seats — as many as Meretz and only two more than Hadash — she will be a junior partner in a government dominated by politicians with whose worldview she fundamentally disagrees. For what? For the post of justice minister, which she held from 2004 to 2006, and the flimsy prospect of advancing peace with the Palestinians.

Besides constituting a betrayal of Livni’s voters, who supported her and her new Hatnua party believing she’d represent an alternative to Netanyahu, Livni’s decision could turn out to be a coalition game-changer. Firstly, it compromises the political alliance between the Jewish Home and the Yesh Atid parties, which agreed not to help Netanyahu build a government without a pledge from the prime minister that he will seek substantial progress on the question of universal conscription.

So far, Yesh Atid’s Yair Lapid and Jewish Home’s Naftali Bennett have been firm in their stance not to join the coalition until Netanyahu agrees to changes the current conscription laws, ensuring that Haredim will have to shoulder an equal part of the national burden. Yet Livni — who likes to present herself as a figure of integrity and principle — joins the government before any such commitments have been made, weakening the bargaining power of these principled holdouts. Things are moving ahead; the Netanyahu-Livni deal signals to Bennett and Lapid that they better not miss out on the chance to grab a seat at the government table.

Her entry also changes the coalition arithmetic. With Livni on board, and Shaul Mofaz’s two-seat Kadima likely to follow, Netanyahu can build a stable coalition majority with only one of the allied Jewish Home and Yesh Atid parties: Likud-Beytenu 31; Shas 11; United Torah Judaism 7; Hatnua 6 — that’s 55 seats in the bag, and Kadima makes 57. Either Yesh Atid (19) or Jewish Home (12) makes for a comfortable majority. Without Livni and Mofaz, Netanyahu would really have needed both Naftali Bennett’s Jewish Home and Yair Lapid’s Yesh Atid, and both were spurning his advances without real commitments on the Haredi draft. Who knows, Netanyahu may now even try to bring in Labor again, and leave both Bennett and Lapid in the cold, or at least threaten to do so.

It was Livni who called the loudest for the three major center-left parties to work together to prevent Netanyahu from retaining the premiership. “There is a battle to be fought. The fight isn’t over yet. We mustn’t surrender,” Livni said on January 4, on live TV, begging Lapid and Labor’s Shelly Yachimovich to enter some sort of alliance against Netanyahu’s Likud-Beytenu. The goal, she asserted, was to “work together to bring down Netanyahu.”

With her agreement to join a Likud-Beytenu-led government in exchange for the justice and the environmental protection ministries (and some other minor perks) as well as heading the government’s peace negotiations — though obviously subordinate to the prime minister — Livni is doing the exact opposite of what she said she wanted to do when (re)-entering politics ahead of the elections. Instead of bringing Netanyahu down — or at least enabling Bennett and Lapid to wrench genuine concessions regarding the Haredi draft from him — she is throwing him a lifeline without forcing him to compromise.

Lapid has made it clear that he will not enter the government unless it adopts his plan for universal conscription, and reportedly said recently he might be able to oust Netanyahu after 18 months in opposition. Labor’s Shelly Yachimovich has said she won’t be a fig-leaf for Netanyahu’s economic policies — but if offered the Finance Ministry and some kind of influence in socioeconomic matters, she might reconsider. Netanyahu really seems to dislike Bennett, but ideologically Likud and Jewish Home are relatively close and if the prime minister sees no other way to build a coalition he might decide to swallow the bitter pill and make his former bureau chief an offer good enough to break his alliance with Lapid.

This last option seems to be the most probable outcome. Lapid is the least likely to give in and join the government: his new party would lose a lot of credit if it folds on the issue dearest to its voters (universal draft), and with 19 seats and its leader’s high visibility, Yesh Atid may prefer to fight from the opposition. Labor, too, will most probably stay out, if only because Netanyahu — a former finance minister with clear ideas about what’s best for Israel’s economy — is not about to hand over the reins of Israel’s fiscal policy to someone with whom he disagrees on almost every issue.

Bennett’s right-wing Orthodox constituency, by contrast, will forgive him if the Haredim get away without a drastic change to Israel’s conscription laws. Bennett and Livni disagree fundamentally on the Palestinian issue — she says a two-state solution is needed urgently; he wants to annex much of the West Bank — but the Jewish Home chief said repeatedly that he would not mind too much if the government restarts peace talks.

Talking can’t hurt, Bennett is probably saying to himself, knowing that as long as Netanyahu is prime minister and dependent on his right-wing coalition partners, no final-status agreement will ever be signed. Anyone slightly familiar with the positions of Netanyahu and Palestinian Authority President Mahmoud Abbas knows it’s just not realistic. And to be on the safe side, Netanyahu reportedly inserted a clause in the coalition agreement with Livni that any peace plan she might bring home will first have to be approved by the Knesset and the government.

It is obvious why Netanyahu inked the deal with Livni: he risked very little (besides angering the more hawkish politicians in his faction) but managed to break the deadlock that hampered his coalition building efforts. With Livni in charge of peace talks, moreover, he can present to the international community a government that seems dedicated to renewing serious negotiations — just in time for US President Barack Obama’s visit.

But why did Livni break her pre-election promises, referring to them as “something being said during election campaigns” in her joint press conference with Netanyahu on Tuesday?

Does she really believe true progress can be made on the Palestinian front, with Netanyahu in the government’s driver seat and Bennett’s hand grabbing for the wheel? She will not even be foreign minister, where the notion that she could achieve something would be slightly more credible. That post is being saved for Avigdor Liberman, who is adamant that the Palestinian problem cannot be resolved at present.

Returning to politics for these elections, after taking a time-out when losing the Kadima leadership last year, Livni promised that this time would be different: She stood for a better Israel, and she wouldn’t quit again if the going got tough.

She’s certainly making a difference; the question is for how long. Netanyahu’s previous unlikely coalition alliance, with Mofaz’s Kadima, lasted about 70 days before collapsing in an orgy of recrimination seven months ago. Tuesday night’s two protagonists will be hoping this match, similarly presented as essential to the national interest, survives rather longer.

The Abba Eban cliche about the Palestinians never missing an opportunity to miss an opportunity has a variant for Livni, much quoted by political analysts on Tuesday, who appears never to miss an opportunity to make the wrong political decision. As one political insider put it, “She’s signed her political death warrant… for about the fourth time.”