but Gamelin considered that, even if it were a false alarm, this would be an excellent opportunity to pressure the Belgians into allowing a French advance into their country. Gamelin intended to execute a decisive offensive against Germany in 1941 through the Low Countries; their neutrality would however, be an obstacle to these plans. If this invasion scare would make the Belgians take the side of France and Britain, this awkward problem would be partially solved and strategically vital ground from which to launch the attack effortlessly gained. On the other hand, if Germany really went ahead with the invasion, it was very desirable that the French forces could entrench themselves in central Belgium before the enemy arrived. Both to intensify the crisis and to be ready for any occasion that presented itself, Gamelin thus ordered 1st Army Group and the adjoining Third Army to march towards the Belgian frontier. The warning by Sas That their deception plan seemed to prove that the documents were genuine, that day further increased Belgian anxiety; the next day they became convinced the situation was critical. In the evening of 13 January, a message from Colonel Georges Goethals, Belgium's Military Attaché in Berlin, included these words: "Were there tactical orders or parts of them on the Malines plane? A sincere informer, whose credibility may be contested, claims that this plane was carrying plans from Berlin to Cologne in relation to the attack on the West. Because these plans have fallen into Belgian hands, the attack will happen tomorrow to pre-empt countermeasures. I make explicit reservations about this message, that I do not consider reliable, but which it is my duty to report." The "sincere informer" was the Dutch Military Attaché in Berlin Gijsbertus Sas who spoke with Goethals at about 17:00; his information always had to be carefully considered because he was in contact with a German intelligence officer who was an opponent of the Nazi regime, known today to have been Colonel Hans Oster. General Van Overstraeten, the King of Belgium's military adviser, who was informed of the message at about 20:00, was astonished that the informant appeared to know about the capture of the plans. It had not been mentioned in any press report of the crash. It was possible that it was part of a grand German deception plan, but equally possible that it was genuine. Acting on the assumption that