LONDON — In the weeks after ISIS operatives struck Paris in November 2015, the group released a prerecorded video of the killers. They stared into the camera, waved serrated knives, raged at the West and specifically warned Britain: You’re next. Footage showed scenes of London through a gunsight.

For the next 13 months, ISIS and those inspired by the group killed and maimed in Brussels, Berlin, Nice and Normandy as well as across the Atlantic in California and Florida. Yet the rhetoric against Britain began to feel like the frothy threats made by the group toward other countries that had avoided attacks, including Iran: loud and menacing but ultimately empty.

Until now. The strikes in the past week against the capitals of the United Kingdom and Iran followed back-to-back attacks in recent months in Britain, by an assailant who used an S.U.V. to smash into pedestrians on Westminster Bridge in March and a suicide bomber at a pop concert in Manchester in May.

From a publicity standpoint, the attacks in Britain and Iran are a lift to ISIS as it loses ground steadily in Syria, Iraq and Libya. Some analysts have interpreted the strikes as a bid by the group to demonstrate its resilience, even as its territory-holding caliphate slowly disappears.

But a review of court records and statements by officials suggests that the violence in London and Tehran was more than just a message. It reflected persistent efforts by the Islamic State since its rise in 2014 to hit targets once thought unassailable — especially in Britain. During this period, officials there intercepted and foiled more than a dozen plots, including five in the past three months.

The number of disrupted plots appears to be far greater in Iran, a country loathed by ISIS which has aimed to hit Iran since at least 2007. A day after the deadly assault last week on the Parliament building and the tomb of Iran’s revolutionary founder, Iranian intelligence officials said they had thwarted 100 terrorist plots in the past two years.

Hours after the violence in Iran, ISIS released its glossy, online magazine, directly challenging skeptics who have questioned the group’s stamina as its territory shrinks. “What many of these analysts failed to admit, however, is that losing territory was nothing new for ISIS,” the article said, referring to the group’s near-defeat in 2011 at the hands of United States forces in Iraq.

“The reality faced by the Crusaders today is that despite their claims that ISIS has been weakened,” it said, “strikes in the heart of the Crusaders’ strongholds in the West will continue to occur just as suddenly and unexpectedly.”

While few details have been shared with the public about thwarted attacks in Iran, plots neutralized in Britain show how ISIS’ reach grew with each attempt. The techniques used by the frustrated attackers, the types of targets they chose and the kind of coaching they received broadly follow the arc of the group’s evolution as it struck repeatedly elsewhere in Europe.

The earliest instances involved would-be attackers who had ideological affinity for ISIS but no direct communication with the group. They took their direction from the many YouTube videos they had watched showing ISIS atrocities, and from detailed instructions delivered online by Abu Muhammad al-Adnani, the group’s spokesman.

The messages initially called recruits to travel to the group’s territory, but as those journeys became more perilous, Mr. Adnani exhorted adherents to stay put and wage violence at home.

Initially seen as the acts of disaffected youth with no real connection to ISIS, these plots did not at first cause alarm, officials say. But in the space of a year, they began to morph into something more complex, as young perpetrators began making online contact with the militant group.

“The rules of engagement changed,” said Max Hill, a British barrister who has prosecuted several high-profile terrorism cases and is now the country’s independent reviewer of antiterrorism legislation.

“We moved from a traditional understanding that there needs to be a trainer — a radicalizer — who is physically present into an era where there may be some remote encouragement coupled with fast access to radical material,” Mr. Hill said. “Those factors alone are enough.”

Just months after ISIS declared its territory-holding caliphate in 2014, the authorities began to see suspects who were in contact with British nationals who had traveled to join the militants.

Such was the case for three men who lived near Southall, a suburb of London, and who maintained contact with a friend who had slipped across the Syrian border, according to court records.

While their friend joined the ranks of ISIS, the three remained stuck in Britain, where they increasingly steeped themselves in the group’s propaganda. In September 2014, when the militants’ spokesman, Mr. Adnani, broadcast an audio recording inciting followers to kill in any way possible.

The three began obsessively shopping for knives, discussing different models in chats punctuated by the sharing of graphic videos.

The men were arrested in November 2014, after two of them were seen walking into a Kitchen Ideas store, where they bought a Victorinox knife and a sharpener, according to documents from their trial.

It is unclear if their contact inside ISIS played a role in their progression toward violence, or if he merely acted as a point of inspiration. But by 2015, investigation records reveal, plotters began tapping the inner core of a network of ISIS online coaches. These coaches spent their days sitting behind computer screens in Raqqa, the Syrian city that is the group’s de facto capital, moving the impressionable men they met on the internet like chess pieces.

They were part of an organized unit nicknamed “The Legion” by the F.B.I., which began tracking them and picking them off in American drone strikes when it became apparent how involved they had become in remotely inciting attacks, including in the United States, Canada, Australia and Britain. One of its best-known members was Junaid Hussain, a British citizen who was among ISIS operatives believed to have been in touch with one of the men involved in the first successful ISIS attack in America, messaging the shooter as he drove to a community center in Garland, Tex., in May 2015, where he would open fire.

(The New York Times)