It's actually not that complicated. The US intelligence services relied on high-level sources, essentially the military's plans for invasion, to come to its conclusions. European intelligence services tended to rely on low-level sources (the status of the units in question) instead. It turns out that the units were, even on the eve of the invasion, simply not ready for an invasion, and the European intelligence had sussed that out. In effect, when the US reported to the Europeans that Russia was preparing for war, the Europeans went "With what army? This one clearly ain't ready for war."
Combine those contradictory signals with the preconceptions that people had. The US intelligence had badly misfired during the Iraq War. There is a (not entirely undeserved) tendency to view the US as excessively warmongering. Putin had a (mostly undeserved) reputation of being a skillful and crafty manipulator. And Russia engaging in naked territorial aggression would require painful reassessments of 30 years of Russia policy (not least of which is the degree to which Europe depended on Russian gas). With all of that weighing against believing the US intelligence, it should be no surprise that Europeans did so.
It's actually not that complicated. The US intelligence services relied on high-level sources, essentially the military's plans for invasion, to come to its conclusions. European intelligence services tended to rely on low-level sources (the status of the units in question) instead. It turns out that the units were, even on the eve of the invasion, simply not ready for an invasion, and the European intelligence had sussed that out. In effect, when the US reported to the Europeans that Russia was preparing for war, the Europeans went "With what army? This one clearly ain't ready for war."
Combine those contradictory signals with the preconceptions that people had. The US intelligence had badly misfired during the Iraq War. There is a (not entirely undeserved) tendency to view the US as excessively warmongering. Putin had a (mostly undeserved) reputation of being a skillful and crafty manipulator. And Russia engaging in naked territorial aggression would require painful reassessments of 30 years of Russia policy (not least of which is the degree to which Europe depended on Russian gas). With all of that weighing against believing the US intelligence, it should be no surprise that Europeans did so.