The Senate on Wednesday voted 97-1 to override President Obama's veto of a controversial bill, known as the Justice Against Sponsors of Terrorism Act (JASTA), that would allow the families of 9/11 victims to sue the government of Saudi Arabia for its alleged financial support of al-Qaeda.

And then things got weird.

Almost immediately after the vote, 28 senators who had just voted for the bill sent a letter to the bill's Senate sponsors, Republican John Cornyn of Texas and Democrat Charles Schumer of New York, saying they were concerned about the "potential unintended consequences that may result from this legislation for the national security and foreign policy of the United States."

The Obama administration has long argued that the bill could end up putting the United States at risk of being similarly prosecuted in foreign courts by undermining a long-standing tradition in foreign relations known as "sovereign immunity." They made this argument when the bill was first up for a vote back in May and promised to veto it if it passed. Then, when it did pass, Obama vetoed it, citing once again his argument for why he thought the bill was a bad idea.

When Congress announced it would hold a vote to override the veto, Obama wrote a letter to Senate Minority Leader Harry Reid, yet again making the case against the bill. But Congress voted to override the veto anyway — the first time they'd ever done so in Obama's entire presidency.

It was only after that final vote on Wednesday to override the veto that Congress apparently figured out that — uh oh! — there might be some negative consequences to the bill they had just voted into law.

Their excuse for why they'd passed this potentially harmful bill? The Obama administration never told them it was a bad idea.

"Nobody really had focused on the potential downside in terms of our international relationships," Senate Majority Leader Mitch McConnell, R-KY, said. "I think it was just a ball dropped."

Never mind the fact that the Obama administration most definitely did explain, again and again, why they thought the bill was a bad idea, the fact is that it's Congress's job to understand the potential impact of any legislation they pass.

As White House Press Secretary Josh Earnest aptly put it, "what's true in elementary school is true in the United States Congress, ignorance is not an excuse, particularly when it comes to our national security and the safety and security of our diplomats and our service members."

Regardless, Congress still seems a little confused about all of this. Luckily, we have an explainer that should clear everything up for them — and you. Here, then, is the Obama administration's case against the JASTA bill, and why top national security experts think he's right.

The background: Making it easier for the little guy to punish powerful governments for supporting terrorism

A group of 9/11 victims' families — as well as some companies who had insured businesses that were damaged in the attacks — have been trying for years to sue the government of Saudi Arabia. They allege that the country helped to finance al-Qaeda and thus should be made to pay financial damages for the group's September 2001 attacks.

But they've been unable to get American courts to agree to hear the case.

This is in large part because of a 1976 US law, the Foreign Sovereign Immunities Act (FSIA), that gives foreign governments immunity from prosecution in US courts.

There are some exceptions to this immunity, but they're very narrow, and the lawyers for the 9/11 victims' families have failed in appeal after appeal to convince the courts that any of the exceptions apply in this case. The JASTA bill would widen these exceptions.

There's already a terrorism exception in the FSIA, but it only applies to countries that have been officially designated by the US Department of State as "state sponsors of terror." That list only includes Syria, Iran, and Sudan — not Saudi Arabia.

This is why the family of Steven Sotloff, the American journalist who was murdered by ISIS, is able to sue the Syrian government, which they allege provided support to ISIS and is therefore partly liable for Sotloff's death.

JASTA supporters argue that the bill would widen the exceptions just enough to allow this one lawsuit against Saudi Arabia to go through.

John Bellinger, a legal advisor for George W. Bush's State Department and the National Security Council, says the bill is much broader than that.

The bill "would remove the immunity not just of Saudi Arabia but of any country in the future that commits the torts or acts of international terrorism specified in the legislation," Bellinger, who is now at the Council on Foreign Relations, told me.

In other words, the JASTA bill would make it easier for US citizens harmed in future terror attacks to sue any foreign government that they could accuse of supporting those attacks.

The bill could put the US government at risk of foreign prosecution

Critics of JASTA, including the Obama administration, have repeatedly argued that passing the bill would weaken the international norm of sovereign immunity, whereby countries block their citizens from suing foreign governments. They warned that weakening this norm could make it more likely that other countries would pass sovereign immunity exceptions allowing their citizens to sue the United States.

In an op-ed in the New York Times opposing JASTA, Duke law professor Curtis Bradley and Harvard law professor Jack Goldsmith, both Bush administration alumni, explained, "If the United States reduces the immunity it accords to other nations, it exposes itself to an equivalent reduction in its own immunity abroad."

"A nation’s immunity from lawsuits in the courts of another nation is a fundamental tenet of international law," they wrote. "Were the sovereign immunity rule to weaken, the United States would be subject to many more lawsuits in foreign courts than any other nation and would become an attractive and high-profile target for politicized litigation designed to contest its foreign policy."

To be clear, the concern isn't that if JASTA becomes law, Saudi Arabia will necessarily retaliate and allow its citizens to sue the American government. Nor is it the case that the global norm against sovereign immunity will likely collapse due to this one law.

Rather, the argument is more second-order: Passing JASTA makes it easier to pass the next sovereign immunity exception and the next one, and eventually some countries may reciprocate. And even if the principle of foreign sovereign immunity doesn't collapse entirely, you're going to have more holes in it and thus more opportunities for the US government to get sued for its actions around the world.

"If you breach a state's sovereign immunity, then the argument against your own sovereign immunity being breached is weaker," Daveed Gartenstein-Ross, a senior fellow at the Foundation for Defense of Democracies, said.

As one hypothetical example, the Iraqi government could pass a law allowing its citizens to sue the US government for damages they suffered during the Iraq war. And if the US lost the lawsuit in the Iraqi courts, Gartenstein-Ross explained, the Iraqi government would legally be able to seize US assets in the country to pay the victims.

This is why the Saudi foreign minister told the Obama administration and Congress (all the way back in April, when the bill was first up for a vote) that Saudi Arabia would have to pull its assets out of the US if the bill were to pass, to prevent those assets from being seized in possible lawsuits against the Saudi government.

"It's actually entirely rational," Gartenstein-Ross said of Saudi Arabia threatening to remove its assets. "If Iraq passed a law saying that Iraqi citizens could sue us for things that happened during the course of the Iraq War, we would get our assets out in a second."

When is a lawsuit an act of foreign policy?

There is something else complicating this issue: For an individual to sue a foreign government is both an individual act and also an intervention into the realm of international relations, which is typically the reserve of states.

If the government of Saudi Arabia, for example, is called before a US courtroom or made to pay damages to a US citizen, then that has an impact on the foreign relations between those two countries.

Critics of JASTA warn that the bill, or any breach of sovereign immunity, thus could potentially allow individual citizens or the US judicial system the power to influence foreign policy, even unintentionally.

In our system of government, most of the power to make foreign policy decisions rests with the executive branch, with some power also given to the legislative branch.

"[I]f you think about our tripartite system of government, sovereign immunity holds that foreign policy is conducted largely, though obviously not entirely, by the executive," Gartenstein-Ross said.

"[O]f the three branches, the courts have the least ability to make foreign policy. They don't have expertise there," he went on. "Their litigation is for an entirely different reason than trying to craft good foreign policy."

This is not to say that if a foreign government supports acts of terrorism against the United States, then this government should be shielded from consequence. Rather, it's that the best way to respond is through the realm of foreign policy — economic sanctions, military retaliation — so that US foreign policy bodies can calibrate the response in a way that will best serve US interests. Allowing individuals to sue foreign governments gives up some of that control.

"From a functional perspective, the argument would be that, yes, states committing acts of terrorism is an awful thing, and in general, the way that the US should deal with that is through the executive undertaking its foreign policy, as opposed to US citizens suing that state in an American court," says Gartenstein-Ross. "It’s something that we prefer the executive to deal with, as opposed to citizens suing in court, independent of any sort of foreign policy imperatives."

While families of terror attack victims have every reason to want to punish a country they believe supported that attack, that may not necessarily be in the best interests of the United States — particularly given doubts, in this particular case, that the foreign government in question is actually guilty.

Making it easier for individual US citizens to sue foreign governments risks introducing a degree of chaos into the foreign policy decision-making process, undermining the ability of the president and the others we've collectively decided as a country to task with such things to craft careful, informed foreign policy on behalf of all Americans.