The outbreak of hostilities between the United States, Israel and Iran on 28 February has catalysed a sharp rise in anti-Semitic violence and terror plots across several Western democracies, leaving Jewish communities from London to Texas on high alert. In the weeks since the first air strikes, security services have recorded a cascade of arsons, foiled conspiracies and criminal convictions, all targeting Jewish worshippers and institutions. Synagogues worldwide moved swiftly to harden their perimeters, yet a pattern of youth radicalisation is testing the limits of early-warning systems.

Nowhere has the climate of fear been more palpable than in north-west London, where a spate of arson attacks has struck synagogues and Jewish charities since late March. The initial attack saw ambulances belonging to the Hatzola volunteer emergency service torched in Golders Green; later, the Kenton United Synagogue and the premises of a Jewish charity were set ablaze. Last week, the Finchley Reform Synagogue, a building that also houses a nursery and is guarded by towering metal fences, was targeted. No casualties have been reported, but analysts in London note that the sequence – none of which has yet been claimed – has driven an already anxious community to a state of hyper-vigilance, forcing a conversation about whether existing police patrols are adequate.

On the continent, French counter-terrorism authorities are contending with the radicalisation of minor-age suspects. In the Rhône region, a 15-year-old boy was arrested on 17 April and subsequently indicted for plotting a “violent action” against Jewish or homosexual communities. The youth admitted he sought to “die a martyr”, according to the national anti-terrorist prosecutor’s office. Paris-based officials stress that the case underscores how extremist propaganda – disseminated via encrypted online channels – continues to attract adolescents who blend jihadist rhetoric with broader grievance narratives, often leaving a faint digital trail that is discovered only after a credible threat has formed.

From Ottawa, a guilty verdict handed down this week in Ontario Superior Court marks the culmination of a parallel investigation. A 17-year-old was found guilty of plotting to “kill as many Jewish persons as possible”. Israel’s consul general in Toronto observed that the verdict validated the Jewish community’s early warnings, long dismissed as alarmist. The teenager had researched explosives and gathered ball bearings and acetone; a co-accused is now standing trial. Viewed from Washington, the most recent alarm originated in North Carolina and Texas, where two young people were arrested in an alleged conspiracy to strike a Houston synagogue by driving through the congregation. Angelina Han Hicks, 18, was detained and is held on a $10 million bond. The plot, which authorities say was designed to maximise Jewish fatalities, follows a separate Detroit-area incident in which an armed man rammed his pick-up truck into a major synagogue.

The geographical spread of these cases – from a London nursery to a French teenager’s bedroom, from an Ottawa basement to a Texas suburb – suggests that the war has acted not simply as a trigger for organised networks but as an accelerant for a diffuse ecosystem of online radicalisation. Multiple capitals privately worry that the real-time spectacle of war, amplified on social media, lowers the threshold for violence among the very young. As investigations unfold, the challenge for intelligence and law enforcement agencies will be to distinguish between aspirational chatter and imminent plots without eroding civil liberties. For now, Jewish institutions across the democratic world are reinforcing their defences, bracing for what a weary security analyst in London called “a long season of menace”.