The withdrawal of Western troops from Afghanistan is the "biggest debacle" that NATO has suffered since its founding, the head of German Chancellor Angela Merkel's party said Monday.

With the Taliban sweeping to power after NATO troops withdrew, CDU party chief Armin Laschet said: "It is evident that this engagement of the international community was not successful. 

"It is the biggest debacle that NATO has suffered since its founding, and we're standing before an epochal change."

Laschet, who is the CDU's candidate to succeed Merkel as chancellor in September elections, said focus at the moment must be on the German military's evacuation operation from Afghanistan.

"But we will talk about the causes and conclusions drawn after this rescue mission -- a no holds barred analysis of errors in Germany, with our allies and in the international community," he said.