It was important for Napoleon to strike a blow before this junction could be made. He resolved to cross the Danube in the suburbs of the capital itself, and attack the Austrians before they were reinforced. In the vicinity of Vienna the channel of the river is broken by many islets. At the island of Lobau, the point chosen for the attempt, the river is broad and deep, but Lobau is separated from the opposite bank by only a narrow branch, while two smaller islets offered themselves as aids in the construction of bridges, there being four channels, over each of which a bridge was thrown. [=The Bridges over the Danube=] The work was a difficult one. The Danube, swollen by the melting snows, imperilled the bridges, erected with difficulty and braced by insufficient cordage. But despite this peril the crossing began, and on May 20th Marshal Massena reached the other side and posted his troops in the two villages of Aspern and Essling, and along a deep ditch that connected them. As yet only the vanguard of the Austrians had arrived. Other corps soon appeared, and by the afternoon of the 21st the entire army, from 70,000 to 80,000 strong, faced the French, still only half their number, and in a position of extreme peril, for the bridge over the main channel of the river had broken during the night, and the crossing was cut off in its midst. Napoleon, however, was straining every nerve to repair the bridge, and Massena and Lannes, in command of the advance, fought like men fighting for their lives. The Archduke Charles, the ablest soldier Napoleon had yet encountered, hurled his troops in masses upon Aspern, which covered the bridge to Lobau. Several times it was taken and retaken, but the French held on with a death grip, all the strength of the Austrians seeming insufficient to break the hold of Lannes upon Essling. An advance in force, which nearly cut the communication between the two villages, was checked by an impetuous cavalry charge, and night fell, leaving the situation unchanged. At dawn of the next day more than 70,000 French had crossed the stream; Marshal Davout's corps, with part of the artillery and most of the ammunition, being still on the right bank. At this critical moment the large bridge, against which the Austrians had sent fireships, boats laden with stone and other floating missiles, broke for the third time, and the engineers of the French army were again forced to the most strenuous and hasty exertions for its repair. [=The Great Struggle of Essling and Aspern=] [=Napoleon Forced to his First Retreat=]