ISTANBUL — The sudden decision by President Trump to endorse Turkey’s move to send its troops into Syria and pull out most of the American forces posted there seems to have shocked the country’s political and military establishment. American analysts and policymakers see events of the past week as largely benefiting Russia, the Syrian government, the remnants of the Islamic State and Iran.

Amid the great clamor of commentary, the United States and Europe are erroneously banking on sanctions on Turkey to contain the fallout. The harsh truth is that the United States, Europe and Turkey share responsibility for the creation of this crisis. They have all made a series of policy mistakes since the beginning of the Syrian war in 2011.

For the United States, the main failure was to naïvely believe that the partnership established with the People’s Protection Units, or the Y.P.G. — an organic offshoot of the Kurdistan Workers’ Party, or the P.K.K., which is considered by Ankara, Washington and Brussels to be a terrorist group — could be long-lived. Oblivious to the huge and negative impact of this commitment on bilateral ties with Turkey, the White House followed the Pentagon’s recommendation based on its an assessment that the Turkish counteroffer, of a group of Syrian opposition fighters trained by both Turkey and the C.I.A., would be inadequate to carry the fight against the Islamic State.

Nevertheless, Turks never came to terms with the United States siding so clearly with a group they consider to be a core national security threat. And American support for the Y.P.G. pushed it toward a perilous overreach. Kurds constitute about 10 percent of Syria’s population but with American support, they came to control almost one-third of its territory.