When Russia took control of Crimea earlier this year, it didn't just openly march its army into the Ukrainian territory.
Instead, it sent personnel in unmarked uniforms to take control of key facilities ahead of a referendum that led to annexation of the strategic peninsula.
Likewise, Chinese fishermen, not the People's Liberation Army, instigated clashes with the Philippines over disputed islands in the South China Sea in 2012.
These sorts of incidents, along with cyber attacks, are in what's called the "gray zone." Eiichi Katahara, an academic at Japan's National Institute for Defense Studies, says such situations can't be defined as either purely peacetime events or military contingencies and therefore create quandaries on how -- or whether -- to respond.
It's a question that the U.S. could face against an increasingly expansionist and provocative China, which also recently had a standoff with Vietnam over an oil rig and has a number of territorial disputes with other countries, including close U.S. allies.
The fact that America and its allies lack defined tactics for defusing many gray-zone situations makes them all the more dangerous, and it's clear that more lie in the future, each with its own nuances and ambiguities.
Michael O'Hanlon, a defense analyst at the Brookings Institution in Washington, said Putin's attempts to operate in the gray zone weren't subtle.
"He got his initial prize (Crimea) but at great expense, and nobody was fooled," he said.
If China tried something similar -- for example, using troops disguised as fishermen in an effort to seize control of disputed islands -- it likely wouldn't be difficult to figure out, too, he said.
"The bad news is that, with this kind of treachery, a state can sometimes get a head start in accomplishing its aims and create facts on the ground that are subsequently difficult to change," he said.
Katahara wrote in the East Asia Strategic Review 2014 that China's recent coercive actions have fueled concern about "the possible increase and protraction of gray-zone situations ... surrounding territory, sovereignty and maritime economic interests, and thus Japan and other neighbors are being pushed to take effective responses."
Katahara's report focuses on Northeast Asia where, U.S. officials say, China has been pushing the envelope hard on what it can get away with to change the status quo, but Russia's actions in Ukraine suggest that the gray zone could come into play almost anywhere.
Gray-zone tactics have been around since ancient times, O'Hanlon said.
"I think back to the perhaps apocryphal case of the 47 samurai who disguised themselves for years as loyal court subjects until attacking in revenge for a previous act of violence against their home village," he said of an incident that recently was the subject of the Keanu Reeves' film "47 Ronin."
The U.S. has operated in the gray zone to achieve its own ends, O'Hanlon said.
"The history of covert operations is a history of this sort of thing, I'd argue," he said. "The CIA and comparable agencies around the world have been in this business themselves in some ways."
In his report, Katahara cites incursions by Chinese government ships and aircraft into the waters around and airspace above the Senkaku Islands, which are administered by Japan but claimed by China and Taiwan.
The incursions run the risks of collisions and accidents, he said.
"In separate incidents in January 2013, Chinese warships locked their fire control radar on a destroyer and a destroyer-based helicopter operated by the Japan Maritime Self-Defense Force," he wrote.
The following November, China announced an "East China Sea Air Defense Identification Zone" that included the Senkakus as if the region were part of Chinese territory.
The Korean peninsula also has been the site of gray-zone incidents.
The flare-ups, each with the potential for escalating, have ranged from axe attacks on U.S. personnel in the Demilitarized Zone to rocket launches, underground nuclear tests and a March 2010 torpedo attack that sunk South Korean navy ship Cheonan, killing dozens of sailors.
However, officials are aware of the potential risks posed by gray-zone incidents and appear to be developing strategies to deal with them.
The U.S. and Japan held talks earlier this year on how to respond. Tokyo wants Washington to join in drafting scenarios for how the two allies would respond in specific cases, but U.S. officials don't want to provoke China by being too specific, according to experts quoted in a Reuters news agency report.
Ralph Cossa, of Hawaii's Pacific Forum think tank, said whether an incident is in the gray zone may be in the eye of the beholder.
"It could be something done with plausible, or implausible, deniability ... or it could be actions that do not invoke the alliance but cause threats," he said, citing the Cheonan sinking as an example.
To date the U.S. hasn't followed a set plan in responding to gray-zone incidents although the U.S. and South Korea have developed contingency plans in case of a repeat of the 2010 shelling of Yeonpyeong Island by the North. Japan wants similar efforts made ahead of any flare-up in the Senkakus, Cossa said.
He suggested establishing hotlines that can be used to defuse a situation before it spins out of control, and making the consequences of gray-zone actions clear in advance.