Washington — A week after the attacks early last year in Paris against the magazine Charlie Hebdo and a kosher grocery, Belgian police officers were fired on as they executed a search warrant in the town of Verviers. Officials learned that the assailants were members of a terrorist cell that had been planning a significant attack on Belgian police officers or civilians.

The incident changed the way counterterrorism officials perceived the Islamic State threat in Europe and made clear that Belgium itself had a greater problem on its hands than it realized.

Before the plot was disrupted, the United States Department of Homeland Security would later explain, nearly all of about a dozen Islamic State plots and attacks in the West had involved lone assailants or small groups. But, the report presciently warned, “the involvement of a large number of operatives and group leaders based in multiple countries in future ISIL-linked plotting could create significant obstacles in the detection and disruption” of new plots.

Indeed, that is now the case, and as the investigation of the even more brutal November attacks in Paris showed, Belgium is a major source of the threat. The attacks on Tuesday in Brussels raised the most serious questions about how prepared the nation was for that threat.