The headline says “probably” because the ruling was on a preliminary injunction. He’ll issue a ruling on a permanent injunction after this order is inevitably appealed and he hears further arguments from both sides, assuming this issue doesn’t land before the Supreme Court first. A weird fact about the judge in this case: He was nominated to the bench by Dubya on … September 10, 2001. If you support the NSA metadata program, there’s the peg for the obligatory “September 10th mentality” talking point.

Rand Paul is, as you might expect, excited:

I applaud U.S. District Court Judge Richard Leon for protecting our #FourthAmendment rights. MUST-READ: http://t.co/Nuj07BO6lW

— Senator Rand Paul (@RandPaul) December 16, 2013

Here’s the opinion, which is worth skimming even if you’re not trained in law. Leon writes more accessibly than most judges, especially in the key section. Three issues in a case like this. One: Doesn’t the law say that only the FISA Court can hear a challenge to the NSA surveillance program? Leon’s a district court judge, not a FISA judge. How can he have jurisdiction? Two: Didn’t the Supreme Court rule once before, in the late 70s, that there’s no privacy interest in phone metadata? If that’s the case, how can there be a Fourth Amendment issue here? And three: Even if there’s a Fourth Amendment issue, why doesn’t mass metadata collection qualify as a “reasonable” search and seizure? It’s crucial to catching terrorists before they strike. Isn’t it?

On point one, Leon says it’s true — he has no jurisdiction to hear statutory challenges to a DOJ collection order. If you’re claiming that the DOJ exceeded the authority granted to it by Congress in issuing an order, that’s for the FISA Court to decide. Not only that, but American citizens don’t have standing to sue in the FISA Court. Only the recipient of a collection order, like Google or Verizon, can do that. In fact, because it’s illegal for the recipient to reveal the existence of a collection order, American citizens aren’t even supposed to know when a collection order is issued, let alone be able to sue the government. So no, there’s no jurisdiction for a federal district court to hear challenges to an order — on statutory grounds. But what if the challenge is on constitutional grounds, i.e. that an order violates the Fourth Amendment? District courts can hear those challenges, says Leon, because potential constitutional violations are of the utmost importance and Congress never went out of its way to say that that type of challenge should be heard only in the FISA Court too. Fourth Amendment questions are fair game for any lower-level federal court.

Fair enough. But what about point two, that the Supreme Court’s 1979 ruling in Smith v. Maryland established a precedent that metadata can be collected under the Fourth Amendment? Leon’s answer is the guts of the opinion; I suggest skipping to page 43 and reading it yourself. He gives four reasons why Smith shouldn’t control decisions on NSA surveillance. First, Smith dealt with a case where the cops wanted a particular defendant’s phone records for use at trial, not everyone’s phone records to store for years into the future. Second, Smith involved a discrete case, not a formal policy established by the federal government and telecom companies for continuous data-harvesting. Third, technology has advanced so wildly in this area since Smith was decided that it’s foolish to use it as controlling precedent. Mass data harvesting was science fiction in 1979 when “pen registers” were the hot legal topic of the day. Clearly, given concerns about scale and invasiveness, courts should consider the issue anew. And fourth, metadata can tell techies much more now than it could 34 years ago. You’re simply dealing with a more significant privacy interest today than you were in Smith. Result: Yes, this program implicates the Fourth Amendment.

The last chance for NSA defenders is to argue that, all of that aside, the searches are “reasonable” because they’re a speedy way for the feds to sniff out and interrupt terrorist attacks. Just one problem with that, says Leon: The feds have produced no evidence of it. He asked them to show him how the metadata program is helping to catch the bad guys where all other methods have failed and, so far at least, they can’t do it. That’s what triggers the preliminary injunction (which is momentarily stayed while the order is appealed). If the DOJ could show gangbusters results in stopping terrorism from the metadata program, that compelling state interest would affect the balance of equities in granting the injunction. As it is, because they’re offering nothing, Leon treats it as an easy call.

I’ll highlight one passage for you since you’re likely to hear Snowden fans mention it anyway:

In other words, if not for Snowden’s leaks, this case literally might not have happened. Per the Supreme Court’s ruling in the Clapper case earlier this year, you can’t get standing before a federal judge by merely speculating that the NSA is targeting you. You need to show a real likelihood of concrete injury. Right, says Leon — and now, thanks to Snowden’s exposure of PRISM, we’ve got that. The leaker has changed the legal facts on the ground, enough so to make a Fourth Amendment lawsuit possible. That may be the single most tangible change in U.S. surveillance policy to have come from Snowden’s leaking, despite Obama’s endless promises about reform.

If nothing else, let’s hope this is harbinger of federal courts being more generous in stretching standing law to accommodate constitutional suits by private citizens. Separation of powers is fertile ground for that, as you already know.

Update: I tweaked the headline, which originally said the judge had halted metadata collection, because the order is stayed while the appeal plays out. If the feds lose the appeal — which may or may not reach the Supreme Court — then the program will be halted.