It’s the biggest tech case of the year, and maybe the decade. Taking place in a California federal court, the case pits Apple against the U.S. government over control of the iPhone, with terrorism and privacy as the backdrop. The outcome will ripple across the entire technology sector and influence governments around the world.

So who is right? Does the FBI, which wants to force Apple AAPL to override the iPhone’s encryption features, have a legal right to do so? Here’s a plain English guide to the legal issues, including constitutional questions, and what will happen next. (Update: the PR fight got nastier on Friday as newspapers lined up to support Apple).

Why is the federal government suing Apple over the iPhone?

The government is frustrated with new iPhone security features that make it near impossible for the FBI or Apple or anyone else (except the phone owner) to crack the password. Now, the feds are asking a federal judge in California to force Apple to write special software that will override those encryption features in order to peer into the iPhone used by one of the San Bernardino terrorists.

The technical facts of the case are important because the FBI wants to go beyond what it has done before. It’s different than forcing a phone company to place a wiretap or making an Internet company hand over emails. If you want to understand the encryption issue — including what Apple can and can’t do to unlock a phone — read this very well-written blog post.

Also note the Justice Department isn’t fighting over just this iPhone. Law enforcement agencies across the country are being thwarted by Apple’s encrypted devices, and the FBI likely chose this case—which involves an infamous terrorist—as its best chance to force Apple to change course.

Why is the U.S. using a 227-year-old law to make Apple obey?

The Justice Department already has a search warrant, but the warrant is doing little good. That’s because Apple says the iPhone’s encryption features mean it can’t access the phone or provide the information the FBI is seeking. In response, the government is citing a 1789 law called the “All Writs Act” to force Apple to comply.

The law itself isn’t as strange as it sounds. According to Stephen Scott, a constitutional law professor at McGill University, the original point of the law was to ensure courts in colonial America had the same traditional powers as those in England. Many of those powers came in the form of old writs (you can think of writs as “royal commands”) we still use today: writs of habeas corpus, certiorari, and so on.

The Justice Department does not refer to a specific type of writ in its court petition (it just cites the All Writs Act), but that doesn’t really matter. The feds just want the court, like courts have on many other occasions, to use its power under the Act to get Apple to comply with the search warrant. But, as noted below, there are two catches.

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Why does Apple think the “All Writs Act” doesn’t apply?

Apple hasn’t filed its legal brief yet (it will in the next few days) so we don’t know for sure, but we can anticipate its objections. The first of these has to do with limits put on the All Writs Act by the Supreme Court.

Specifically, the court won’t order Apple to act if it concludes the company is “so far removed from the underlying controversy” or if would place an “unreasonable burden” on Apple. It’s a safe bet Apple will make the “burden” argument but, according to lawyer Alexander Abdo of the ACLU, the company could also claim it’s too far removed for the investigation:

“The All Writs Act doesn’t allow government to conscript a company into service if the company doesn’t have the information … If the FBI is doing an investigation, it can’t force the local locksmith to help it break into a house,” said Abdo.

So a court might find the limits on the All Writs Act apply to Apple. Or it might conclude the Act doesn’t apply at all—that’s what a U.S. magistrate judge in Brooklyn suggested last year in another case involving iPhone encryption. Specifically, the judge pointed the Justice Department to another law called CALEA, which is about assisting law enforcement, and suggested the feds couldn’t fall back on the older law instead. (The Brooklyn judge hasn’t issued a final ruling, but his preliminary comments could help Apple).

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What About the Constitution?

No matter what the All Writs Act means, the Justice Department will not be able to make Apple comply if doing so would violate the Constitution. This opens another can of worms.

First, there is the Fourth Amendment’s search and seizure rules. While the FBI obtained a valid warrant, it must still execute the search in a reasonable manner—and forcing Apple to break encryption might not qualify. According to Abdo of the ACLU, Apple has a claim based on the Fifth Amendment too: it could argue that the process would deprive it of liberty without due process of law.

Finally, law professor Ryan Calo of the UW School of Law suggested on Twitter that the FBI’s proposed order violates free speech rights. The reason is that forcing Apple to write new software may amount to “compelled speech” that contravenes the First Amendment.

So Who Will Win—Apple or the FBI?

It’s way too early to say. For now, it’s clear that Apple has compelling legal arguments to support its claim that it shouldn’t be forced to decrypt the phone, but that doesn’t mean it will prevail. Meanwhile, the Justice Department, as it points out in its brief, has used the All Writs Act to prevail in numerous other cases, including ones where companies had to turn over passwords.

What happens next?

Apple will submit its legal brief to the California magistrate judge in the next few days (update: it now reportedly has till next Friday). A ruling will come weeks, or possibly months, later. Then the appeals will begin: to a district judge, to the 9th Circuit Court of Appeals and, in all likelihood, to the Supreme Court. If that occurs, the case probably won’t get resolved till 2017 at the earliest.

In the meantime, look for an intense PR war to keep brewing. On Tuesday, an open letter by Apple CEO Tim Cook warned customers that the FBI demand was a threat to data security and would set a dangerous precedent. Then, on Thursday, the While House tried to assuage fears by saying it only wanted to decrypt the one device, and did not want Apple to “create a new backdoor to its products.”

Meanwhile, Apple’s nominal rivals in Silicon Valley — including Google GOOG and Facebook FB — are signaling support for the iPhone maker. As for the Justice Department, it has Donald Trump on its side, who called Apple “disgraceful” on Wednesday.