The December 2000 Deadline For Signing up to the ICC was important

Nations had until December 2000 to sign up if they wanted to participate in the discussions about the ICC. Not signing would have meant that while that nation would still be subject to it, they would not have been able to participate in decisions at future ICC discussions.

After the deadline to sign, countries can ratify but no longer sign the treaty. A country that had failed to sign would lose influence during negotiations to decide how the court will work.

US and others had signed last minute to the ICC in December 2000

With the looming deadline to sign the International Criminal Court at the end of December 2000, the United States signed, as did some other nations, so that it would allow them to be involved in future negotiations about this treaty.

The U.S. had voiced concerns about their soldiers being possibly tried for war crimes but the ratification of this major new international institution didn't get much media coverage as Danny Schechter describes (in the previous link), so the possibility of broad debate had been further minimized.

Because the U.S. government has no intention of ratifying the court's treaty anytime soon, it has focused on the supposed outrage that the court would have jurisdiction over the citizens of a state that has not ratified the treaty. But it is common practice for a government to prosecute a foreign national for crimes committed on its territory without first seeking permission of the foreigner's government. The jurisdiction of the ICC amounts to no more than a delegation of this widely accepted power for the most serious human rights crimes. Indeed, Washington itself routinely exercises far more expansive jurisdiction in unilaterally pursuing alleged terrorists or drug traffickers even when their crimes were not committed on U.S. soil.

— A Selective U.S. Vision of Justice, Human Rights Watch World Report 2000.

(By May 2002, the Bush Administration unsigned, thus withdrawing from the ICC, citing unfounded concerns regarding sovereignty, as discussed on previous pages on this site.)