It's beginning. The legal system's constitutional defence against United States President Donald Trump's policies won its first battle last weekend when several federal district courts blocked the implementation of his executive order banning immigration from seven Muslim-majority countries.

The lawyers' resistance won't always succeed and it will face many bumps in the road over the next four years. The resistance got a big lift this time from Mr Trump himself, who ignored the Department of Justice and its first-rate lawyers. Eventually, Mr Trump may figure out how to use the legal system to achieve his goals instead of working against it.

But the pattern set in the nearly 48 hours after the executive order was issued is likely to recur: Such overreach from the Oval Office will be met by rapid-fire, grassroots opposition from the legal side of civil society.

The courts are going to be sympathetic; Judges don't like to see the legal system being treated cavalierly. The most important lesson of the opening salvo in what will soon be a full-blown war is that the Constitution and laws of the US are a formidable bulwark against the arbitrary exercise of presidential power.

As I wrote at about 3 am on Nov 9, "it's all about the Constitution now".

Mr Trump may believe that he has a popular mandate and that it authorises him to take bold, unilateral action without regard to existing laws. But he's wrong. The rule of law is designed to block him from circumventing existing procedures and principles.

Mr Trump's confusion is perhaps understandable, even if not forgivable. He got elected by breaking the rules, after all. So maybe he thinks he can govern that way too.

But unlike the unwritten rules of campaigning that he flouted, the legal rules are written down. And they come with a full-blown ecosystem of institutions and actors, not to mention the courts that are profoundly committed to sustaining law itself.

Mr Trump and his team seem truly not to understand that. Astonishingly, they did not go through the ordinary and almost mandatory procedures that take place within the government before executive orders with the force of law are issued. This amounted to flouting the legal checks that exist within the executive branch itself.

First, they did not run the executive order past lawyers at the Department of Justice, whose Office of Legal Counsel reviews all presidential actions that are potentially unlawful. Even the George W. Bush administration in the fevered aftermath of the Sept 11 attacks managed to submit its plans for interrogation to the Office of Legal Counsel (where the so-called torture memos came from).

The Office of Legal Counsel would've caught most or all of the legal vulnerabilities that plague Mr Trump's order and that led the various district courts to enjoin it. So it wasn't just wrong for the administration to ignore the Office of Legal Counsel. It was also counterproductive.

Then there is the bizarre fact that the administration did not prepare Justice Department lawyers to be able to defend its order by giving them the chance to prepare arguments in advance. Reportedly, in various courts around the country, the department's lawyers couldn't answer basic questions about the order or its practical effects - which means they essentially couldn't defend it. A better defence - or any - might have saved the order.

In practice, the administration failed to give the US Customs and Border Protection guidance on what the order aimed to do. That led to the kind of embarrassing chaos that helped the courts see the whole thing as an exercise in lawlessness.

Also potentially very damaging to Mr Trump is the epic green-card screw-up. The order never mentioned lawful permanent residents and it might have omitted them altogether. It took hours for the administration to announce that they were included.

Then it appeared to reverse itself - after causing panic for perhaps as many as 500,000 lawful residents from the seven countries on the list.

The whole episode is also sure to cause grave concern for more than 13 million green-card holders from the rest of the world. To hold a green card was once to feel safe leaving the US, secure in the knowledge you'll be able to get back in. No longer.

The green-card decision is thus another consequential instance of Mr Trump sabotaging himself by failing to get good legal advice in advance. The American Civil Liberties Union was ready to swing into action the moment the order took effect. The courts listened.

Read closely, the order seemed to say that anyone with a valid visa who landed in a US airport could not be sent home. That's a broader holding than the court would have made had the Department of Justice mounted a good defence.

Mr Trump can appeal the injunction, and he likely will. The federal appellate courts will get into the game and maybe the US Supreme Court. We've seen only the kick-off and the return, not even the first few minutes of what will be a long game.

But the legal momentum is already against Mr Trump acting unilaterally. That's good for the rule of law - for now.

BLOOMBERG VIEW

• The writer is a professor of constitutional and international law at Harvard University.