President Donald Trump's decision to leave the Joint Comprehensive Plan of Action (JCPOA), also known as the Iran nuclear deal, has raised tensions between the US and Iran.

But the chance of any diplomatic recovery to the situation ended on Thursday when the US killed Iranian Maj. Gen. Qassem Soleimani in an airstrike.

Trump made a mistake by killing Soleimani because the move could bolster support for Iranian hardliners and lead to a major counterstrike by Tehran.

Brett Bruen was the director of global engagement in the Obama White House and a career American diplomat. He runs the crisis-communications agency Global Situation Room.

Visit Business Insider's homepage for more stories.

Diplomacy died Thursday night outside Baghdad International Airport.

Despite President Donald Trump's worrisome withdrawal from the Iran nuclear deal, many world leaders believed a negotiated solution was still possible. At the very least, they hoped a costly conflict could be avoided.

But after the American strike that killed Iranian Maj. Gen. Qassem Soleimani, their worst fears may come to pass. The US has crashed straight into a serious confrontation with Tehran.

There are few outside of Iran and its network of proxies who mourn the passing of Soleimani. He was a brutal military mind and brought terror to countries across the Middle East. Unfortunately, killing him won't bring peace or advance American interests. His death will lead to even more lives being lost and further embolden the hardliners in Iran.

It will likely cause great consternation in the ornate halls at Mar-a-Lago, but President Barack Obama actually did far more than Trump to debilitate Soleimani. There was evidence that moderates finally gained ground after the 2015 nuclear deal and made progress pushing reforms within the country. By no means was it perfect. Yet the changes were noticeable and, above all, nonnuclear.

Iran continued to covertly support militias and meddle in other countries' affairs. That being said, such extracurricular activities are pretty much par for course in their neighborhood. Saudi Arabia and other gulf nations engage in comparable surreptitious support for groups aligned with their interest.

Nonetheless, the US and European powers had at last won significant leverage over Tehran. There were checks in the deal on their most disruptive behavior, and it made sure to keep their nuclear development from getting too far out of hand.

By contrast, ditching the deal significantly strengthened hardliners such as Soleimani. He was able to engineer large-scale attacks in Saudi Arabia, Syria, and Iraq. Even so, by most accounts Iranian diplomats were deflated but not yet defeated.

Both sides engaged in a delicate dance over the past couple of years. The Trump administration sought to strangle the flow of funds to Tehran. The Iranians attempted to retaliate by inflicting higher costs, both military and economic, on America and its allies.

Each continued to subtly indicate they wanted to find a way out of the impasse. Neither was ready to make the first move. In no small part this was because Soleimani and his more radical wing had become much more empowered.

The strike's real consequences

The death of diplomacy with Iran is significant, not just sentimental. Its demise ushers in a period of unrelenting unrest. American officials traveling around the world now have a target on their backs. They will have to limit their engagements and exposure. The costs for protecting many more people will skyrocket.

We can also expect to see a significant response. Already Iran announced that it would no longer comply with any of the limits laid out in the 2015 nuclear deal, and there's likely more to come.

Tehran will not just come against our embassies and military bases. American businesses and organizations are extremely vulnerable. Iran knows very well how to use unconventional weapons such as cyberattacks to rattle stock markets, and companies will now have to pay considerable costs to protect their supply chains worldwide.

For decades, Tehran has prepared plans and put in place the pieces that would be needed to strike back when needed against an American attack. Those now just need to be activated and executed.

It is not at all clear that Trump has a strategy for how we effectively defend against those myriad threats, let alone how he expects to solve this situation. While undoubtedly military planners will have drawn up options, they would require a direction and discipline that the president has yet to show.

When Trump pulled us out of the Iran deal, I called it the end of the American era. No longer would the US serve as the guarantor of global security and stability. No longer could we be counted on to honor our word. Despite my pessimism, what was difficult to imagine at the time was that the US would so quickly become the instigators of international instability.

For a man who is always seeking to claim titles and tie himself to extraordinary accomplishments, this one may finally be worth of his ambition: an error of epic proportions.

Bruen is president of the crisis-communications firm Global Situation Room Inc. He was director of global engagement at the White House and spent 12 years as an American diplomat. In addition to teaching crisis management at Georgetown University, he serves on the board of the FDR Foundation at Harvard and the Clinton Institute at University College Dublin.