There was no diplomacy because the position of Russia was completely ignored. Surprisingly China stated openly that it was the fault of the west. That is ridiculous because they are not the people to invade here, but at no point was there effective diplomacy. On the contrary, everyone called Putins hand and now he put it on the table.

It failed because the US diplomats were too ignorant to take Putin's "red line" demands (Ukraine never getting into NATO) seriously and they discarded any possibility for immediate negotiations on that matter too eagerly.