Meantime, however, storms were gathering on various parts of the horizon, and all the enemies of France appeared to be striving to prevent her achieving her political unity. Threatening preparations for the relief of La Rochelle were going forward in the English ports; Wallenstein was carrying all before him in Germany, and the fainting princes of the North were sending despairing appeals for assistance; while, worst of all, the Spaniards from the Milanese and the Duke of Savoy had invaded the duchy of Mantua and the marquisate of Montferrato, to which Charles of Gonzaga, Duc de Nevers, had succeeded on the death of Vincenzo I, Duke of Mantua, in 1627, and were threatening Casale, on the Po, a fortress which it was of the most vital importance to France to save from falling into unfriendly hands.[112] But, until La Rochelle was taken, France could do little or nothing to aid her hard-pressed ally, for all the troops which could be spared from the defence of the frontiers, save those engaged to hold Rohan and the Huguenots of the South in check, were concentrated before the Protestant stronghold; all the money which could be raised was being thrown into the mud of its canal. Recognising the impossibility of abandoning the siege, but sorely troubled by the news from Italy, Richelieu determined to make an attempt to take La Rochelle by surprise, although he was well aware that his chance of success was of the slightest. On March 11, accordingly, he sent for Bassompierre and informed him that that night he was sending Marillac to endeavour to blow up the Porte des Salines, and instructed the marshal to have 2,000 foot and 300 horse in readiness to support him. Bassompierre assembled his troops with all due secrecy at the place appointed, where he was joined by the Cardinal, with a force about equal to his own. They waited there all night, expecting every moment to hear the sound of the explosion; but nothing happened, and it subsequently transpired that Marillac and the men who were carrying the petard had lost their way in the darkness. In the early morning of the 13th another attempt was made, this time on the south-eastern side of the town; but it failed completely, and more than forty men were killed and wounded. After this second fiasco, Richelieu prudently abandoned the idea of taking La Rochelle by a _coup de main_, and, feeling very uneasy as to what was happening in Paris, wrote to the King pressing him to hasten his return to the army, in order to discuss with him the situation.