FOR WEEKS German officials had fretted about the risk of a terrorist attack on the country’s Christmas markets: crowded, difficult-to-secure gatherings with religious connotations that could make them attractive targets for jihadists. In 2016 a Tunisian migrant aligned with Islamic State drove a lorry into a Christmas market in west Berlin, killing 13 people in one of modern Germany’s deadliest terrorist incidents. Since then, patrons of the markets had grown wearily familiar with the presence of security barriers and armed police.