Postwar assessment of whether the Maginot Line served its purpose has been mixed. Its enormous cost and its failure to prevent German forces from invading France have caused journalists and political commentators to remain divided on whether the line was worthwhile. The historian Clayton Donnell commented, "If one believes the Maginot Line was built for the primary purpose of stopping a German invasion of France, most will consider it a massive failure and a waste of money... in reality, the line was not built to be the ultimate savior of France". Donnell argued that the primary purpose of "prevent[ing] a concerted attack on France through the traditional invasion routes and to permit time for the mobilization of troops... was fulfilled", as was the French strategy of forcing the Germans to enter Belgium, which ideally would have allowed "the French to fight on favorable terrain". However, he noted that the French failed to use the line as the basis for an offensive. Marc Romanych and Martin Rupp highlight that "poor decisions and missed opportunities" plagued the line and point to its purpose of conserving manpower: "about 20 per cent of [France's] field divisions remained inactive along the Maginot Line". Belgium was overrun, and British and French forces evacuated at Dunkirk. They argue had those troops been moved north, "it is possible that Heeresgruppe A's advance could have been blunted, giving time for Groupe d'armees 1 to reorganize". Kaufmann and Kaufmann commented, "When all is said and done, the Maginot Line did not fail to accomplish its original mission... it provided a shield that bought time for the army to mobilize... [and] concentrate its best troops along the Belgian border to engage the enemy."