It took George W. Bush and Barack Obama a while to warm up to each other. They had many differences—in party, in age, in temperament, in style. Obama had risen to the presidency in part by peddling a harsh critique of Bush’s administration. The relationship grew gradually over time. The two men joked at the unveiling of Bush’s White House portrait in 2012. Bush invited Obama to the opening of his presidential library. By the time Michelle Obama and the former president embraced at the opening of the National Museum of African American History, stories emerged about the odd friendship between the couples.

That growing warmth was fostered in part by a detente between the two men. While Obama fired broadsides against Bush on the campaign trail, Bush mostly shrugged it off. He instructed Treasury Secretary Hank Paulson to keep Obama briefed on responses to the economic crisis, Jonathan Alter reported, with Paulson deeming Obama far more informed about the economy than John McCain. During the transition process, Bush invited Obama and his national-security appointees to war games.

After Obama’s inauguration, Bush quietly left the scene and mostly avoided talking about politics. He repeatedly stressed the importance of allowing Obama to govern without the interference of an ex-president. The silence was so striking that when reports surfaced in April 2015, seven years into Obama’s presidency, that Bush had privately criticized Obama’s ISIS policy, it was headline news. Just as notably, former Bush spokesman Ari Fleischer disputed the reports.  “He never mentioned Obama. He gave direct, blunt answers to the hottest topics of the day involving politics of the Middle East,” Fleischer told CNN.

Obama, in turn, responded to Bush’s withdrawal using the same method—he seldom mentioned Bush’s name. As conservatives did not fail to point out, whenever Obama was confronted with his administration’s struggles to get the economy rolling, he complained that he had been handed an extremely poor economy. But he usually avoided saying just who he had inherited that economy from. It was a small courtesy for the former president, and a token of Obama’s gratitude for Bush’s graciousness. Former Obama Chief of Staff Bill Daley told The Washington Post that Obama didn’t mention Bush much in private, either, though some of his staffers grumbled about the former president. (Many of Bush’s aides still found Obama’s criticisms of their old boss unfair and distasteful.)

The public truce between Obama and Bush was notable because of the harsh tone of the 2008 campaign, but it followed the pattern set by previous commanders in chief: The outgoing president would stay out of the way and the incoming president would avoid attacking him. Despite Barack Obama’s attempts to build a rapport with Donald Trump during the presidential transition, and despite Trump’s public gratitude, the tradition seems moribund now.

Obama had already declared his intention to deviate from tradition “if there are issues that have less to do with the specifics of some legislative proposal or battle, but go to core questions about our values and our ideals.” He has already broken his silence once, with a spokesman issuing a statement on protests last weekend over Trump’s immigration executive order. “President Obama is heartened by the level of engagement taking place in communities around the country,” the statement said, calling the demonstrations “exactly what we expect to see when American values are at stake.”

But if Obama is willing to fire a broadside at his successor,  Trump’s administration has shown its willingness to attack Obama in terms that are equally harsh, or even harsher. In a statement on Wednesday about Iran conducting a ballistic-missile test, National Security Adviser Michael Flynn spent nearly as much ink blasting Obama’s policies as he did the Iranians:

The Obama Administration failed to respond adequately to Tehran's malign actions—including weapons transfers, support for terrorism, and other violations of international norms. The Trump Administration condemns such actions by Iran that undermine security, prosperity, and stability throughout and beyond the Middle East and place American lives at risk. President Trump has severely criticized the various agreements reached between Iran and the Obama Administration, as well as the United Nations—as being weak and ineffective.

On Thursday, Press Secretary Sean Spicer also opened up on Obama. Spicer was being quizzed about a phone call between Trump and Australian Prime Minister Malcolm Turnbull, which reportedly ended acrimoniously in part due to differences over an agreement by the Obama administration to accept 1,250 refugees from Australia.

“The president is unbelievably disappointed in the previous administration’s deal that was made and how poorly it was crafted, and the threat to national security it put the United States on,” Spicer said, a statement remarkable not only for its directness but for the accusation that Obama had endangered American security.

A showdown between presidents is unpredictable because it’s so rare. But Obama might feel emboldened by his public standing. He has a hefty advantage in approval ratings—he left office with a 59 percent approval rate, according to Gallup, against Trump’s current 45 percent. (Incidentally, he also had the upper hand when he entered office, with two-thirds of Americans approving of his performance against just 34 percent approval for Bush, which might have encouraged Bush to stay mum.)

Nonetheless, these are likely only the opening skirmishes of a longer campaign of sniping between Obama and Trump. Trump’s agenda is full of just the kinds of items that Obama said would force him to speak up. The tone of Flynn’s attack on Obama startled White House reporters, who asked Spicer on Wednesday whether to expect more like that. Yes, came the answer.

“I think in areas where there's going to be a sharp difference, in particular national security, in contrasting the policies that this president is seeking to make the country safer, stronger, more prosperous, he's going to draw those distinctions and contrast out,” Spicer said. “And so he's going to continue to make sure that the American people know that some of these deals and things that were left by the previous administration, that he wants to make very clear what his position is and his opposition to them. And the action and the notice that he put Iran on today is something that is important, because I think the American people voted on change.”

One change they voted on, whether they realized it or not, was the end of the tradition of comity between former and current presidents.