With so great a disparity of force he could not offer battle, but in every way he harassed and interrupted the advance of the Imperialists, while he sent pressing messages to Oxenstiern for men and money, and to Marshal Horn, who commanded in Alsace, to beg him march with all haste to his assistance.
Unfortunately Horn and Duke Bernhard were men of extremely different temperaments.

The latter was vivacious, enterprising, and daring even to rashness, ready to undertake any enterprise which offered the smallest hope of success.

Marshal Horn, on the other hand, although a good general, was slow, over cautious and hesitating, and would never move until his plans appeared to promise almost a certainty of success.
Besides this, Horn, a Swede, was a little jealous that Duke Bernhard, a German, should be placed in the position of general-in-chief, and this feeling no doubt tended to increase his caution and to delay his action.
Consequently he was so long a time before he obeyed the pressing messages sent by the duke, that Ratisbon, after a valiant defence, surrendered on the 29th of July, before he had effected a junction with the duke's army.

The Imperialists then marched upon Donauworth, and this place, after a feeble defence, also capitulated.

The duke, heartbroken at seeing the conquests, which had been effected at so great a loss of life and treasure, wrested from his hands while he was unable to strike a blow to save them, in despair marched away to Swabia to meet the slowly advancing army of Marshal Horn.
No sooner was the junction effected than he turned quickly back and reached the vicinity of Nordlingen, only to find the enemy already there before him, and posted on the more advanced of the two heights which dominate the plain.