He had early successes negotiating with the Danes in border disputes and helped to secure Sigismund III Vasa's accession to the Polish throne.

During the discussions of Sigismund's election, Polish and Swedish tensions heightened after a misunderstanding about the annexation of Estonia, a fault that was blamed on Sparre.

He was effectively utilized as commissioner of negotiations with the Danes in March 1575 regarding the Swedish–Danish border before later being dispatched to the courts of the Count of East Frisia and the Holy Roman Emperor in the summer of the same year.

In March 1576, Sparre and other members of the nobility submitted new proposals for the liturgy on behalf of the king and the appointment of higher officials to Duke Charles, which he rejected.

[17] Sparre's patronage included part of the printing costs of his work, namely his history of the Nordic countries, Chronicon Saxoniae et vicini orbis arctoi ('The Chronical of Saxony and [Those] Near the Arctic World'), published in 1590.

[28] However, dukes served simply as a dominium utile, an extension of the rights and privileges of the king without the same claims to sovereignty, which applied only to "accidents and conveniences" (tillhörigheter och nyttigheter).

[42] The Statute provided sweeping guarantees to Sweden's independence, secured a prohibition against Polish interference, and unequivocally defined Estonia as "a Swedish province".

[44][c] These promises were later rescinded from the official compact after negotiations between John and Sigismund, the latter declaring at his coronation that Sweden would never be forced to cede any territory to the Commonwealth,[44] though Sparre continued to be blamed for being willing to sacrifice Estonia for the rest of his life.

[48] The election of Sigismund was fraught with issues, but Sparre and his allies saw a political union with the Polish–Lithuanian Commonwealth as a way of getting aid to Sweden, which was then drawing close to a war with Russia.

[48] By 1589, Sigismund was looking to abdicate – assuming he could find a suitable candidate to take his place – and John was losing interest in legal power, seeking to reestablish a familial relationship with his son and return him to Sweden.

[50] On 5 August, eight Privy Councilors – including Sparre – accompanied John to Reval with a small entourage of reluctant Polish senators following three weeks later, with no significant armed force.

[51] When the events in Reval turned out to be a disaster, it remained difficult to dissuade John, who had become obsessed with reestablishing a filial relationship with Sigismund.

On 5 September, fifty members of the nobility and eight army commanders signed a petition "castigating his failings as a ruler in terms of extraordinary harshness", appealing to him to abandon the plan.

When this failed, ten days later the Privy Councilors attempted again to dissuade John, adding in large text in all capital letters to the bottom VINCAT AFFECTUM RATIO ('May reason triumph over passion').

The Polish entourage exhorted Sigismund to return to Warsaw as an invasion of Podolia was underway by the Tartars and the Privy Councilors of Sweden were threatening to leave Reval as well.

[53] The events in Reval turned John away from his support of legislative strength and pushed him away from Sparre and his allies; the king had never theretofore capitulated to the nobility.

[59] Sigismund repeatedly intervened in defense of Sparre and his allies and even Anne of Poland attempted to clear him of all charges relating to the abandonment of Estonia.

[61] While in prison, he complained in letters that he had been unable to attend his father-in-law's funeral, that his brother had been imprisoned, and that he was not a heretic in spite of accusations to the contrary.

[64] On 17 November 1592, John III died, forgiving all the men who opposed him in Reval on his deathbed in exchange for their promise to remain faithful to Sigismund.

[72] Prior to Sigismund's accession, the nobility and Protestant clergy of Sweden made certain demands, called the Postulata nobilium (lit.

Where is now the rich and powerful nobility, the ornament and glory of the kingdom, which aforetime could ride its hundreds to court, to the service and honor of king and country?

They too are seen no more: a great part are fallen upon the edge of the sword during so many years against so many and various enemies; some has the aforsaid tyrant [Erik XIV] persecuted and done to death; and they who survive are much impoverished.

[80] Sigismund's rule in Sweden was extremely unpopular; he was perceived to have walked back several promises,[82] such as installing Catholics in the government – which served to further alienate him from an increasingly Protestant nation – and failing to institute the agreements made in the Postulata.

[85] Sparre initially considered an armed revolt to disrupt Charles's rise to power, but quickly determined that this was an untenable option.

[86][87] On 20 March 1597, Sparre and Erik Stenbock fled for Denmark before making their way to Poland, with Sten Banér and Jöran Posse [sv] following them in May.

[89] In December, Sparre was dispatched to visit the Dukes of Mecklenburg and Pomerania to secure their neutrality in the upcoming conflict before heading to Denmark to persuade Christian IV to invervene on behalf of Sigismund.

[91][92] Further, Christian agreed to allow Polish troops to pass through if it became necessary and offered to serve as a mediator who would appear impartial, but ultimately side with Sigismund.

[92] In an undated letter to Nils Gyllenstierna [sv] shortly before his death, Sparre complained that no one visited him, wrote to him, or gave him advice or comfort.