FILE - In this June 16, 2021, file photo President Joe Biden meets with Russian President Vladimir Putin in Geneva, Switzerland. (AP Photo/Patrick Semansky, File)

What do rocket strikes by Shiite militias in Iraq, ransomware attacks on targets in the U.S., and Russia's use of mercenaries on battlefields in the Middle East have in common? They are part of a trend in which America's rivals are using nonstate actors and quasi-deniable means to put pressure on its interests.

Washington is frequently finding itself on the business end of a classic strategy — proxy warfare — for which it has yet to devise an effective answer.

Proxy warfare has been around forever. Today, America is the target of this approach.

Russian criminal organizations have allegedly carried out cyberattacks against America's critical infrastructure, most notably through the ransomware attack that shut down Colonial Pipeline earlier this year. The Kremlin's connection to these elements is murky, but it seems unlikely that Putin would tolerate the attacks if he didn't believe they were advantageous to the Russian state.

The allure of proxy attacks is that they offer impact with (relative) impunity. Iran can use Iraqi militias to weaken the U.S. position in Iraq, or gain leverage in nuclear negotiations, without having to openly attack a superpower. Russian criminal groups can foment disorder within the U.S. without fully revealing the Kremlin's hand. And the harder attribution is, the harder it has traditionally been for Washington to justify a sharp, punishing response.

Proxy attacks thus offer America's rivals the ability to coerce the U.S. within limits: They are a classic "gray zone" tactic used to exert pressure short of war. At the same time, they offer countries such as Russia and Iran an opportunity to test the approaches — massive cyberattacks, large-scale violence against U.S. targets in the Middle East — they might employ if a bigger fight broke out.

So far, Washington has found it hard to formulate winning countermeasures. Proportional retaliation against the proxies themselves — pinprick airstrikes against Iranian-backed militias; arrests or financial reprisals against Russian criminal groups — doesn't appear to unduly trouble their government sponsors. This is largely because it allows Tehran and Moscow to maintain the myth of deniability and deflect the costs onto more expendable actors. The obvious alternative for Washington is to hit back harder — and be willing to go after the sponsor as well as the proxy.

In 2018, the Department of Defense destroyed a contingent of 200 Russian mercenaries that got too close for comfort in Syria. In early 2020, President Donald Trump ordered the killing of Qassem Soleimani, the commander of Iran's Quds Force, after his Iraqi proxies' attacks on Americans crossed the line.

Similarly, President Joe Biden has reportedly warned Putin that he will retaliate against Russian state interests — perhaps through cyberattacks, perhaps through economic sanctions or other means — if large-scale cyberattacks continue.

The logic is sound. Proxy attacks won't stop until U.S. rivals start to fear that they will suffer more from the American response than they gain from the initial probe. Showing that the U.S. will respond asymmetrically — that it reserves the right to strike back across multiple domains — can inject greater uncertainty into these countries' calculus.

The Biden administration, like the Trump administration, is trying to signal that it is losing patience with proxy attacks. Proving that point may require a degree of sustained reprisal — and sustained confrontation — that will take Biden considerably further than he wants to go.

Hal Brands is the Henry Kissinger Distinguished Professor at Johns Hopkins University's School of Advanced International Studies, and a scholar at the American Enterprise Institute.