These were just a few of what some Israeli and American intelligence officials say were nearly a dozen plots that form the backbone of a continuing offensive by Iran and Hezbollah against Israel and its allies abroad. But the links seem tenuous at times, the tactics variable, the targets scattered across the globe, from the Caucasus to Southeast Asia to the Mediterranean.

“This is not a spy thriller that necessarily has a plot readers can follow from page to page,” said Matthew Levitt, director of the program on counterterrorism and intelligence at the Washington Institute for Near East Policy. “Iran and Hezbollah both thrive on reasonable deniability.”

Analysts say the shadow war pitting Israel against Iran and Hezbollah has more in common with the cloak-and-dagger maneuverings of the C.I.A. and the K.G.B. during the cold war than the publicity-hungry terrorism campaign of Al Qaeda. It represents a return to the idea that the most effective attack is often an ambiguous one.

“They want just enough ambiguity that you can’t nail down that they did it, the seed of doubt that makes it difficult for Israel or the United States to respond,” said Andrew Exum, a senior fellow with the Center for a New American Security in Washington. The undercover conflict signaled “a return to the black arts of the cold war,” he said.

After the blast in Bulgaria, both Iran and Hezbollah denied involvement almost as quickly as Prime Minister Benjamin Netanyahu of Israel pointed the finger at them. American and Bulgarian officials backed the assessment off the record, but would not say so openly. There has been little hard evidence presented to show how or by whom the plots were coordinated.

Israeli intelligence has evidence of many telephone calls between Lebanon and Burgas in the two months before the bombing, according to a senior government official who spoke on the condition of anonymity because the information is classified, with the volume intensifying in the three days leading up to it.

But they are no more prepared to expose the details of their counterintelligence work publicly than the attackers are to claim responsibility. “We know the sources in Lebanon,” though not the identity of those on the other end in Bulgaria, the official said. “They shouldn’t know that we know the numbers in Lebanon.”

Weeks after the attack, the Bulgarian investigation has largely stalled. Officials there have yet to identify the attacker, also killed in the blast, or his suspected accomplices. They are hesitant to declare Hezbollah responsible without ironclad proof, given that the European Union has never designated the group a terrorist organization.

European allies expect more concrete evidence than the volume of calls before taking steps against Hezbollah. They maintain “some skepticism that it was Hezbollah as an organization itself, and not, for instance, Iran using individuals with some Hezbollah affiliation,” said a senior security official in Germany.

The investigation in New Delhi appears further along, but there, too, diplomatic and trading ties leave India with a dilemma. Iran is a major supplier of oil to India, which has struggled to balance relations with Iran, Israel and the United States.

The Indian police issued warrants in March for three Iranian citizens in connection with the New Delhi attack. But when The Times of India recently reported that the police had identified Iran’s Revolutionary Guards as responsible, officials immediately denied the report.

Several of the plots have been hasty and even sloppy. In Thailand, the suspects set off an explosion in their own safe house. Many other suspected plots failed or were disrupted before they materialized. That gave some experts the impression that the assaults were planned in a hurry, perhaps because Iran and Hezbollah scrambled to lash out after a string of covert attacks on Iran’s nuclear program. The civil war in Syria, which threatens the government of a key Iranian ally, may be another spur to action.

“We see Iran and Hezbollah as monoliths without realizing there is internal competition, dissent, factionalism, and these things become important when we have external pressures like we do now,” said Rashmi Singh, a lecturer at the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland. “What’s happening in Syria is making this more acute than ever,” she said.

The proliferation of plots has kept Israel on guard. It was less cat-and-mouse than Whac-A-Mole, with plots popping up in Africa, Europe, South Asia and Southeast Asia.

“There was kind of a desperation to carry out these attacks; they weren’t necessarily as well prepared,” a senior Israeli official said. “Even when they were thwarted there was a sense they’d done something. They need to show some results.”

Analysts say that the increased planning was also evidence of deepening anger in Tehran as international sanctions took hold. Simple revenge is a possible motive, they say.

Hezbollah has sworn to retaliate against Israel for the 2008 assassination of Imad Mughniyah, the group’s former security chief, who was killed in a car bombing in Syria. Iran blames Israel for killing at least four Iranian nuclear scientists, several with magnetic bombs placed on their vehicles. Efraim Halevy, formerly head of the Mossad, conceded that the shadow war was not just one-sided with “a measure of attack on both sides.” He drew a distinction between “innocent bystanders” and “people who are threatening you.”

The Israelis count an assassination attempt against the Israeli consul general in Istanbul and the killing of a Saudi diplomat in Karachi, both in May 2011, as the beginning of the recent offensive. The United States said it had thwarted an Iranian-backed plot last year to kill the Saudi ambassador in Washington with the help of a Mexican drug gang.

But the intensity of the new offensive came into focus in February when tactics similar to those used in the killings of Iranian scientists were deployed against Israelis. A motorcyclist pulled up alongside a vehicle belonging to the Israeli Embassy in New Delhi and attached a small explosive device. The wife of an Israeli defense official was wounded in the ensuing blast, as were three others, but all survived.

In the Georgian capital, Tbilisi, on the same day, a driver for the Israeli Embassy “found something stuck to the car with a magnet,” he told the Georgian television station Real TV. Investigators cordoned off the area while explosives experts defused the bomb. No one was harmed.

The following day, an explosion rocked a rented home in Bangkok. One man lost his legs after a bomb exploded as the Bangkok police tried to arrest him. A second man was arrested by the police at Bangkok’s Suvarnabhumi Airport trying to flee the country. A third suspect sought in the case was apprehended in neighboring Malaysia, where he is still fighting extradition.

The police in Thailand said those suspects, too, were targeting Israeli Embassy staff members with explosives that were fitted with magnets. All three, as well as another man and a woman sought in connection with the case, are Iranian.

The Indian police say that one of their suspects was “in touch” with a suspect in Thailand. Israeli officials say they have cellphone calls and text messages between Thailand, India and Baku, the capital of Azerbaijan. Security services there announced in March that they had arrested 22 people who they said had been hired by Iran to carry out terrorist attacks against Israeli, American and other Western targets.

Ronnie Bar-On, chairman of the Foreign Affairs and Defense Committee in Israel’s Parliament, said Iran could pass weapons, ammunition, technology and “knowledge” through diplomatic mail. “We have unlimited resources, we have unlimited time” to deal with this and other threats, Mr. Bar-On said.

By midsummer, despite the close call in New Delhi, all the operations had been either thwarted or bungled. Two Iranians were arrested in Nairobi in June in connection with the explosives found at the Mombasa golf course. In July, the police in Cyprus arrested a Lebanese man with a Swedish passport suspected of scouting airplanes and buses carrying Israeli tourists to the Mediterranean island.

A few days later, a slender young man with a large backpack slipped into a crowd of Israeli tourists at the airport in Burgas as they lugged baggage from the terminal to a waiting bus. The blast from the bomb hidden in his backpack shattered any sense of invincibility that the Israelis might have felt.

That civilians were the victims in Bulgaria rather than diplomatic or military targets was not necessarily a sign that others were behind that attack. “When you fail and time passes, sometimes you lower your bar,” Mr. Levitt said.

Yoram Schweitzer, a senior research fellow at the Institute for National Security Studies, said those responsible wanted to see Israel embroiled in fighting with its neighbors. “Precisely for this reason it is best for Israel to adopt a restrained policy and respond at a time of its own choosing,” Mr. Schweitzer said, “in a targeted and covert fashion.”

The conflict, in other words, may continue to play out in the shadows.