Under heavy German diplomatic pressure, Prince Paul had Yugoslavia join the Tripartite Pact on 25 March 1941, but with a proviso that Yugoslavia would not grant transit rights to the Wehrmacht to attack Greece. As the Metaxas Line protected the Greek–Bulgarian border, the Wehrmacht generals much preferred the idea of attacking Greece via Yugoslavia instead of Bulgaria. During a hasty meeting of Hitler's staff after the unexpected 27 March Yugoslav coup d'état against the Yugoslav government, orders for the campaign in Yugoslavia were drafted, as well as changes to the plans for Greece. The coup d'état in Belgrade greatly assisted German planning as it allowed the Wehrmacht to plan an invasion of Greece via Yugoslavia. The American historians Allan Millett and Williamson Murray wrote from the Greek perspective, it would have had been better if the Yugoslav coup d'état had not taken place as it would have forced the Wehrmacht to assault the Metaxas Line without the option of outflanking the Metaxas Line by going through Yugoslavia. On 6 April, both Greece and Yugoslavia were to be attacked.