The United States Navy expects a “period of uncertainty” in the Persian Gulf following President Donald Trump’s decision to terminate America’s participation in the Iran nuclear deal, the chief of the U.S. military branch said this week.

“It is a period of uncertainty that we are entering into right, how the whole world will respond to this latest development,” Adm. John Richardson, the chief of U.S. Naval Operations, told reporters, according to Reuters.

“[We have to] remain alert, I mean even a little bit more alert than usual to just be open to any kind of response or new development or something like that,” Richardson added.

Adm. Richardson spoke to reporters after visiting the aircraft carrier USS George H.W. Bush off the coast of Virginia where American and French troops are participating in joint training.

“Richardson said the U.S. Navy had not seen provocative Iranian behavior in the Gulf since Trump’s announcement but was watching closely,” Reuters points out.

Until recently, friction plagued the already tense relationship between the U.S. and Iran navies in the Persian Gulf.

Iran had consistently and dangerously engaged in aggressive behavior against America’s naval presence in the Gulf, at times approaching U.S. ships at high speeds. However, U.S. Gen. Joseph Votel, the top American commander in the Middle East, noted in January that Iran’s military had stopped harassing U.S. naval vessels in the Persian Gulf with their fast boats for at least five months.

Gen. Votel attributed the ongoing respite in harassment to America’s military readiness, noting, “We have messaged our readiness … and that [Iran’s behavior] isn’t tolerable.”

As of March of this year, the U.S. Navy indicated that it had seen a definite “change” in Iran’s behavior.

“It seems like they’ve absolutely made a conscious decision to give us more space,” Cmdr. William Urban, the spokesman for U.S. Naval Forces Central Command, declared, according to CBS News. “That is definitely a change in their behavior.”

There has been no “unsafe and unprofessional” action since August 2017, the U.S. military noted. The U.S. Navy commander failed to speculate on the reasons behind the change.

Now, the U.S. Navy expects Iran to resume its aggressive behavior in the Persian Gulf.

On May 8, President Trump announced the U.S. would exit the Iran nuclear deal and reimpose sanctions on the Islamic Republic.

In explaining the president’s move, U.S. Defense Secretary James Mattis told lawmakers on May 9:

I believe what’s happened is the president could not affirm as required that this agreement was being lived up to. … We have walked away from the JCPOA [Joint Comprehensive Plan of Action] because we found it was inadequate for the long-term effort [against Iran].

Mattis noted that the accord had not stopped Iran from engaging in ongoing “malign activities” in and around the Middle East, further explaining:

We will work with our allies and try to bring Iran back into more responsible behavior, at the same time addressing all five of the threats that Iran constitutes. The nuclear issue which is foremost. Certainly, the terrorism issue that I just cited.

The ballistic missile efforts they have, cyberattacks they’ve been conducting, and then the threats to international commerce whether it be out of the Red Sea where we have seen it most recently or back up in the Persian Gulf where it has relented over the last several months.

In June 2015, U.S.-led world powers — the U.K., France, Germany, Russia, and China — reached the controversial Iran nuclear accord, which was supposed restrict Iran’s nuclear operations in exchange for relieving the Islamic Republic of crippling economic sanctions, namely those the United States and the United Nations have imposed.

Non-U.S. signatories have said they will remain committed to the pact despite America’s withdrawal.

“The withdrawal has upset Washington’s European allies, cast uncertainty over global oil supplies and raised the risk of conflict in the Middle East,” Reuters reports.

“Iran, which sees the Gulf as its backyard and believes it has a legitimate interest in expanding its influence there, has long argued that the region should organize its own security collectively, without outside powers,” it adds.