Less apprehensive of open enemies, than of the jealousy of the friendly powers, he left Upper Germany, which he had secured by conquests and alliances, and set out in person to prevent a total defection of the Lower German states, or, what would have been almost equally ruinous to Sweden, a private alliance among themselves. Offended at the boldness with which the chancellor assumed the direction of affairs, and inwardly exasperated at the thought of being dictated to by a Swedish nobleman, the Elector of Saxony again meditated a dangerous separation from Sweden; and the only question in his mind was, whether he should make full terms with the Emperor, or place himself at the head of the Protestants and form a third party in Germany. Similar ideas were cherished by Duke Ulric of Brunswick, who, indeed, showed them openly enough by forbidding the Swedes from recruiting within his dominions, and inviting the Lower Saxon states to Luneburg, for the purpose of forming a confederacy among themselves. The Elector of Brandenburg, jealous of the influence which Saxony was likely to attain in Lower Germany, alone manifested any zeal for the interests of the Swedish throne, which, in thought, he already destined for his son. At the court of Saxony, Oxenstiern was no doubt honourably received; but, notwithstanding the personal efforts of the Elector of Brandenburg, empty promises of continued friendship were all which he could obtain. With the Duke of Brunswick he was more successful, for with him he ventured to assume a bolder tone. Sweden was at the time in possession of the See of Magdeburg, the bishop of which had the power of assembling the Lower Saxon circle. The chancellor now asserted the rights of the crown, and by this spirited proceeding, put a stop for the present to this dangerous assembly designed by the duke. The main object, however, of his present journey and of his future endeavours, a general confederacy of the Protestants, miscarried entirely, and he was obliged to content himself with some unsteady alliances in the Saxon circles, and with the weaker assistance of Upper Germany.
As the Bavarians were too powerful on the Danube, the assembly of the four Upper Circles, which should have been held at Ulm, was removed to Heilbronn, where deputies of more than twelve cities of the empire, with a brilliant crowd of doctors, counts, and princes, attended. The ambassadors of foreign powers likewise, France, England, and Holland, attended this Congress, at which Oxenstiern appeared in person, with all the splendour of the crown whose representative he was. He himself opened the proceedings, and conducted the deliberations. After receiving from all the assembled estates assurances of unshaken fidelity, perseverance, and unity, he required of them solemnly and formally to declare the Emperor and the league as enemies. But desirable as it was for Sweden to exasperate the ill-feeling between the emperor and the estates into a formal rupture, the latter, on the other hand, were equally indisposed to shut out the possibility of reconciliation, by so decided a step, and to place themselves entirely in the hands of the Swedes. They maintained, that any formal declaration of war was useless and superfluous, where the act would speak for itself, and their firmness on this point silenced at last the chancellor. Warmer disputes arose on the third and principal article of the treaty, concerning the means of prosecuting the war, and the quota which the several states ought to furnish for the support of the army. Oxenstiern's maxim, to throw as much as possible of the common burden on the states, did not suit very well with their determination to give as little as possible. The Swedish chancellor now experienced, what had been felt by thirty emperors before him, to their cost, that of all difficult undertakings, the most difficult was to extort money from the Germans. Instead of granting the necessary sums for the new armies to be raised, they eloquently dwelt upon the calamities occasioned by the former, and demanded relief from the old burdens, when they were required to submit to new. The irritation which the chancellor's demand for money raised among the states, gave rise to a thousand complaints; and the outrages committed by the troops, in their marches and quarters, were dwelt upon with a startling minuteness and truth.
In the service of two absolute monarchs, Oxenstiern had but little opportunity to become accustomed to the formalities and cautious proceedings of republican deliberations, or to bear opposition with patience. Ready to act, the instant the necessity of action was apparent, and inflexible in his resolution, when he had once taken it, he was at a loss to comprehend the inconsistency of most men, who, while they desire the end, are yet averse to the means. Prompt and impetuous by nature, he was so on this occasion from principle; for every thing depended on concealing the weakness of Sweden, under a firm and confident speech, and by assuming the tone of a lawgiver, really to become so. It was nothing wonderful, therefore, if, amidst these interminable discussions with German doctors and deputies, he was entirely out of his sphere, and if the deliberateness which distinguishes the character of the Germans in their public deliberations, had driven him almost to despair. Without respecting a custom, to which even the most powerful of the emperors had been obliged to conform, he rejected all written deliberations which suited so well with the national slowness of resolve. He could not conceive how ten days could be spent in debating a measure, which with himself was decided upon its bare suggestion. Harshly, however, as he treated the States, he found them ready enough to assent to his fourth motion, which concerned himself. When he pointed out the necessity of giving a head and a director to the new confederation, that honour was unanimously assigned to Sweden, and he himself was humbly requested to give to the common cause the benefit of his enlightened experience, and to take upon himself the burden of the supreme command. But in order to prevent his abusing the great powers thus conferred upon him, it was proposed, not without French influence, to appoint a number of overseers, in fact, under the name of assistants, to control the expenditure of the common treasure, and to consult with him as to the levies, marches, and quarterings of the troops. Oxenstiern long and strenuously resisted this limitation of his authority, which could not fail to trammel him in the execution of every enterprise requiring promptitude or secrecy, and at last succeeded, with difficulty, in obtaining so far a modification of it, that his management in affairs of war was to be uncontrolled. The chancellor finally approached the delicate point of the indemnification which Sweden was to expect at the conclusion of the war, from the gratitude of the allies, and flattered himself with the hope that Pomerania, the main object of Sweden, would be assigned to her, and that he would obtain from the provinces, assurances of effectual cooperation in its acquisition. But he could obtain nothing more than a vague assurance, that in a general peace the interests of all parties would be attended to. That on this point, the caution of the estates was not owing to any regard for the constitution of the empire, became manifest from the liberality they evinced towards the chancellor, at the expense of the most sacred laws of the empire. They were ready to grant him the archbishopric of Mentz, (which he already held as a conquest,) and only with difficulty did the French ambassador succeed in preventing a step, which was as impolitic as it was disgraceful. Though on the whole, the result of the congress had fallen far short of Oxenstiern's expectations, he had at least gained for himself and his crown his main object, namely, the direction of the whole confederacy; he had also succeeded in strengthening the bond of union between the four upper circles, and obtained from the states a yearly contribution of two millions and a half of dollars, for the maintenance of the army.
These concessions on the part of the States, demanded some return from Sweden. A few weeks after the death of Gustavus Adolphus, sorrow ended the days of the unfortunate Elector Palatine. For eight months he had swelled the pomp of his protector's court, and expended on it the small remainder of his patrimony. He was, at last, approaching the goal of his wishes, and the prospect of a brighter future was opening, when death deprived him of his protector. But what he regarded as the greatest calamity, was highly favourable to his heirs. Gustavus might venture to delay the restoration of his dominions, or to load the gift with hard conditions; but Oxenstiern, to whom the friendship of England, Holland, and Brandenburg, and the good opinion of the Reformed States were indispensable, felt the necessity of immediately fulfilling the obligations of justice. At this assembly, at Heilbronn, therefore, he engaged to surrender to Frederick's heirs the whole Palatinate, both the part already conquered, and that which remained to be conquered, with the exception of Manheim, which the Swedes were to hold, until they should be indemnified for their expenses. The Chancellor did not confine his liberality to the family of the Palatine alone; the other allied princes received proofs, though at a later period, of the gratitude of Sweden, which, however, she dispensed at little cost to herself.