A dozen Iranian speed boats brazenly swarm U.S. warships in the Persian Gulf. A Russian fighter jet buzzes a U.S. Navy surveillance plane flying over the Mediterranean Sea. North Korea fires a barrage of missiles launched from the air and ground.

All the incidents took place in mid-April. All were mounted by some of America's top adversaries. As coronavirus stalks the globe, sapping attention, budgets and government personnel at home and abroad, U.S. adversaries from Moscow to Pyongyang are flexing their muscles and testing U.S. resolve.

U.S. defense and national security officials said that although America remains on guard for potential threats as it works to overcome the coronavirus in a world of unpredictability, they have not detected extraordinary reasons to sound the alarm.

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President Donald Trump sent out a warning Wednesday on Twitter: "I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea." He offered no additional context.

Defense Secretary Mark Esper told NBC's "Today" last week that "it's hard to say" whether the Iranian and Russian episodes reflected efforts to probe for U.S. vulnerability amid the COVID-19 pandemic that has sickened more than 826,000 Americans.

The Defense Department, he said, considered "this a normal week."

No Americans were killed or injured as a result of the Iranian, Russian and North Korean actions. National security analysts said that the timing of these incidents may not be coincidental but that there is little evidence that America's global opponents are capitalizing on conditions created by the pandemic.

America's enemies "aren't necessarily doing anything different or unusual because of coronavirus," said Joseph Cirincione, president of Ploughshares Fund, a Washington-based global security foundation. "And this thing cuts both ways: All militaries are concerned about others' perception of them during a time of crisis.

"Is the timing suspicious? Maybe. Internal dynamics can also explain these incidents."

To be sure, provocations from U.S. foes and rivals are not new.

On April 15, when Iranian Islamic Revolutionary Guard Corps Navy boats darted past U.S. warships conducting exercises in international waters in the northern Persian Gulf it was the latest example of routine Iranian harassment toward U.S. vessels that has intensified since the Trump administration exited a nuclear deal with Tehran.

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That same day, a Russian SU-35 fighter jet came within 25 feet of a U.S. Navy P-8A Poseidon surveillance plane flying in international airspace, putting the pilots and crew at risk, according to the U.S. Navy. Air Force Gen. Tod Wolters, NATO’s supreme allied commander in Europe, characterized the Russian pilot’s maneuver, including a pass in which he flew upside down, as unprofessional as opposed to incitement.

That, too, was just the most recent incident in a string of encounters between Russian and U.S. jets that raised concerns over the risk of midair collisions over European airspace. A report in 2014 from the European Leadership Network, a London-based think tank, found at least 40 examples that year of dangerous incidents involving Russian and Western militaries in "violations of national airspace, emergency scrambles, narrowly avoided mid-air collisions and close encounters at sea."

North Korea's salvo of suspected cruise missiles toward waters off Japan on April 14 was among the most high-profile missile tests Pyongyang has conducted since nuclear negotiations with the Trump administration stalled.

The tests came on the eve of a North Korean state anniversary and parliamentary elections in neighboring South Korea. Shea Cotton, a nuclear security expert at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies, warned a week earlier in a column for Defense News that North Korea is "signaling this will be its busiest year of missile testing yet." Cotton noted that Kim Jong Un's regime conducted nine missile tests in March, the most in a single month recorded in his research institute's database.

Cirincione said he did not think the U.S. military was more at risk as the Pentagon responds domestically and overseas to the COVID-19 health crisis despite the idling of the USS Theodore Roosevelt, a nuclear-powered aircraft carrier docked in Guam, that has been sidelined because hundreds of its sailors tested positive for the disease.

"We have such an overcapacity of military," he said. "Bases all over the world and an alliance system like no other. Is anybody pressing us at the minute? China or the Russians or even the Islamic State terror group? I just don't see it."

About 3,500 active-duty U.S. military personnel, out of approximately 1.3 million, have tested positive for coronavirus, according to the Defense Department.

A U.S. State Department report published this month said China might be secretly pushing ahead with low-level underground nuclear tests despite its claims that it strictly adheres to an international moratorium on all nuclear tests.

Although Cirincione said unproven allegations that Beijing carries out low-intensity nuclear tests have been "percolating" for many years and nothing indicates China is exploiting a period of U.S. weakness. China, after all, is still grappling with COVID-19 – albeit at a later stage.

Trita Parsi, executive vice president of the Quincy Institute for Responsible Statecraft, a think tank in Washington, said that before coronavirus fully took hold in the USA, the Trump administration, not Iran, appeared to view the outbreak as an opportunity to gain advantage by "amplifying" its maximum pressure strategy – the view that crippling sanctions would force Tehran to choose between its own economic viability and its destabilizing activities around the region.

"Iran has all of these sanctions on it, and on top of it, you have an epidemic that makes it more likely Iran's regime will collapse. As a result, any sanctions relief would be counterproductive," Parsi said, summarizing how he said U.S. officials such as Secretary of State Mike Pompeo approached the Iran issue when Tehran was seeing hundreds of COVID-19 deaths a day and the USA had relatively few.

He said both sides viewed the outbreak as a potential battleground.

Parsi said the global spread of the virus could provide "an excuse and an opportunity for the Iranians to do something really drastic that may have less of a downside – I'm not saying there aren't risks – than under normal circumstances."

One example: In mid-March, the Pentagon launched airstrikes targeting an Iranian-backed Shiite militia group in Iraq suspected in a rocket attack that killed and wounded American and British troops stationed at a base north of Baghdad. At the time, the severity of the coronavirus outbreak was beginning to become clear to the Trump administration. It did not take the strike any further.

"The politics of the outbreak are churning against the backdrop of wider U.S.-Iran animosity that has grown steadily since Tehran decided in May 2019 to counter U.S. 'maximum pressure' with 'maximum resistance,' " the International Crisis Group, a Washington-based think tank, wrote in a research paper in early April.

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A Defense official characterized the Iranian and Russian incidents as "weird," noting that the Russians haven't made such a hazardous maneuver in some time and the exercises in the Persian Gulf had been underway for a week before the Iranians responded. Time will tell if the incidents are isolated or part of a pattern, said the defense official, who was not authorized to speak publicly about them.

Adding to the impression the U.S. is being challenged in some way amid the pandemic: On the same day that Moscow's fighter jet buzzed a U.S. Navy surveillance plane in the Mediterranean Sea, Russia's military tested a series of missiles that U.S. Space Command Gen. Jay Raymond said were capable of "destroying" U.S. satellites in low orbit. On April 22, Iran launched what it described as its first military satellite into orbit, a move that could support its long-range ballistic missile program. Tehran's missiles underpin part of the Trump administration's tensions with Iran because Washington is concerned they could carry nuclear weapons.

Gen. Hossein Salami, the head of Iran’s Revolutionary Guard, said its space program reflected Iran's membership of the "world's most powerful armies."

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The COVID-19 pandemic has forced the Pentagon to shift its footing to remain ready to fight, and troop rotations and some routine logistical operations have been paused.

On April 14, Army Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, said sailors aboard the USS Harry Truman saw their deployment extended to account for the USS Theodore Roosevelt's unscheduled sick time in Guam.

The U.S. Air Force flexed its muscles with an "Elephant Walk" in which B-52 bombers and other jets taxied on the runway at Guam's Andersen Air Force base.

The “Elephant Walk" on April 13 was conducted with joint assets across Guam to showcase U.S. military readiness and "the ability to generate combat airpower at a moment's notice to ensure regional stability throughout the Indo-Pacific," Air Force Lt. Col. Megan Schafer, a spokeswoman for Pacific Air Forces, said in an email.

For various reasons, America's adversaries are also more potentially imperiled.

In North Korea, there are unconfirmed reports Kim Jong Un may be gravely ill after heart surgery. The North Korean leader's health has long been a focus of speculation. He's a heavy smoker and suffers from obesity. South Korea's government downplayed the reports. If Kim did die, it could put the nuclear-armed and isolated country in a fraught security posture with the United States as it dealt with a succession battle and perhaps COVID-19. North Korea has denied any infections.

More than 10% of Iran's lawmakers have fallen ill with coronavirus, far more than anywhere else. The disease has not spared top officials, including its senior vice president, Cabinet ministers, Revolutionary Guard members and Health Ministry officials.

Several Iran lawmakers died from COVID-19.

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One area where the COVID-19 field may be more level is terrorism, according to Colin Clarke, a security expert at the Soufan Center, a New York-based think tank.

Clarke said coronavirus led the United States, France and other countries to withdraw elite forces, mainly trainers, helping combat militant groups around the world, especially in West Africa. But "the militants themselves are also vulnerable. It's not like they can just continue life as usual while the security forces are hampered," he said. "They may use this period to be more reckless, but they are also going to get sick."

Clarke said the coronavirus outbreak highlighted that perhaps too much of America's military and security preoccupations have revolved around terrorism and more conventional military threats from well-known enemies.

"I teach a class where I ask my students to rank the five greatest threats to global security. Global pandemics is one of them. Every year it comes in at No. 5," he said.