The Palestinian Authority becomes the 123rd member of the International Criminal Court today, a major step that could move its decades-long conflict with Israel into a courtroom.

Israel is opposed to the court, is not a member and has no plans to cooperate with investigators already looking into possible crimes by both sides during fighting. It was furious when the Palestinians announced their application on December 31 and tried to undermine it by lobbying to cut funding to the court.

But the Palestinians’ membership poses a major dilemma for Israel. Israel may be stuck on the legal sidelines unless it makes a major policy shift by cooperating with or even joining an institution born out of the principles of the Nuremberg trials of Nazi leaders.

“The ICC can issue summons to appear in court and arrest warrants. It will say a lot about a country if it doesn’t respond,” a Palestinian official said.

“The panic attacks we’ve seen [from Israel]) suggest we are not the only ones to take the court seriously.”

Israeli officials say they will wait to see what happens once the Palestinians have joined the court before commenting.

The benefits of signing up to the treaty that governs the court include diplomatic immunity for government officials working with the court, fast-tracked reviews of cases it puts forward and the right to nominate judges.

From April 1, the court will have automatic jurisdiction over any crimes committed in the territory of the Palestinian Authority, or by its citizens.

It has also already received permission to make a preliminary examination of events since June 13, 2014, shortly before Israel began a military offensive in Gaza.

Several Palestinian-linked NGOs are expected to submit legal arguments and dossiers of evidence to the court in the coming days or weeks, but the state of Palestine is not expected to make a formal referral to the prosecutor, which would trigger the appointment of a panel of judges.

An Israel-Palestine case, experts believe, would examine battlefield conduct by both Israeli troops and Palestinian militants in Gaza, but could also consider the Israeli settlements on occupied land wanted by the Palestinians.

Palestinians seek a state in Gaza, the West Bank and East Jerusalem, lands Israel captured in 1967. Most countries view Israel’s settlement building on occupied land as illegal.

On the flipside, from the moment the Pales­tinians join, prosecutors will gain the right to investigate any alleged crimes on the Palestinian side, even against the will of the Palestinian leadership.

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