Former McLaren boss Eric Boullier said he realised immediately that the McLaren-Honda adventure would be a failure.
"Already in the first meeting I realised how unprepared they were for the enormous challenge they faced," the Frenchman told the Sokuho magazine.
"I told Ron Dennis immediately that we would need at least three or four years of development to get to the top, but the contracts were already signed and Dennis was sure he could relive past successes."
After three years, McLaren dumped Honda and switched to customer Renault power.
Boullier said: "The current engines are very sophisticated, and only Mercedes was ready to outperform the competition based on years of development.
"The union between McLaren and Honda could not have come at a more complicated time in terms of the technical-historical moment."
Boullier said McLaren's woes were then exacerbated with the switch to Renault power, when the team's 2018 chassis was not up to scratch.
"We realised in April that had completely missed the mark with the car," he said.
"We found serious problems in the correlation of data and no solution until it was too late. In some respects we were less competitive than in 2017, which was difficult to cope with on a level of morale."