The Spanish declaration of war was followed by a new treaty of alliance with France, signed in Paris on the 5th of January, 1805, and confirmed on the 18th of the month at Madrid. Spain undertook to furnish, by March 21, to the common cause, at least twenty-five ships-of-the-line and eleven frigates; but the military direction of the whole allied effort was entrusted to Napoleon.

This accession of Spain could not become immediately operative, owing to the backward state of her armaments caused by the previous demands of Great Britain. The emperor therefore adhered for the time to his existing plans, formulated on the 27th and 29th of September. These proving abortive, he next framed, upon lines equal both in boldness and scope to those of the Marengo and Austerlitz campaigns, the immense combination which resulted in Trafalgar.

The events of the ten following months, therefore, have an interest wholly unique, as the development of the only great naval campaign ever planned by this foremost captain of modern times. From his opponents, also, upon whom was thrown the harder task of the defensive, was elicited an exhibition of insight, combination, promptitude, and decision, which showed them to be, on their own element, not unworthy to match with the great emperor. For Napoleon was at this disadvantage, — he could not fully realize the conditions of the sea.