Rifts between Nato allies plagued their efforts to form a police force for Afghanistan, an inquiry heard on Monday.
The US was dissatisfied with the pace of training provided by Germany, the lead instructor for Afghan police officers.
A German police chief, in turn, said America's military-style training was less stringent and lasted a mere two weeks for some recruits.
"We did not welcome the American involvement," said Peter Joerdening, the former head of mission of the German Police Project Team in Afghanistan.
By 2020, the quality of training for Afghan police had declined, he told a committee of German MPs.
There were also widespread reports of corruption in the Afghan police.
The Taliban seized control of Afghanistan in 2021, deposing the western-backed government and ending the 20-year US-led mission in the country.
Afghan security forces were unable to resist the Taliban advance despite the training of 300,000 troops and 80,000 police officers.
The US saw Germany's police training as too ponderous when the security situation remained fragile in Afghanistan.
American trainers stepped in, of whom Mr Joerdening said bore little resemblance to German civilian police.
Afghans had little option but to accept the US training and co-ordination between the Nato allies never recovered, he added.
"The need was there for stabilisation," he said.
"We said, 'OK, perhaps it's possible to have the lowest, front-line rung trained on the American track, while we do the more sustainable form'.
"That was the cognitive dissonance we tried to bridge, which seemed possible at the time, but it dramatically failed."
Another difficulty was that responsibility was split between Germany's police training and Italy's efforts to set up an Afghan justice system, the committee was told.
The UK took on responsibility for counter-narcotics while the US focused on army training, under a division of labour agreed in 2001.
Mr Joerdening said the West's promises to build a stable Afghan state had not been met.
Another witness, Tilmann Roeder, the head of a peacebuilding organisation, said securing peace should have come before nation-building in Afghanistan.
The German inquiry has previously heard that western intelligence was in the dark about the poor state of the Afghan army.
European powers were also kept out of the loop when the US under Donald Trump negotiated a deal with the Taliban, it heard last year.
An inquiry in Britain said the manner of the UK's withdrawal was "a disaster and a betrayal of our allies".
