In which country did King Henry become the reigning monarch during 1573?
Following the death of the Polish ruler Sigismund II Augustus on 7 July 1572, Jean de Monluc was sent as the French envoy to Poland to negotiate the election of Henry to Polish throne in exchange for military support against Russia, diplomatic assistance in dealing with the Ottoman Empire, and financial subsidies.
On 16 May 1573, Polish nobles chose Henry as the first elected monarch of the Polish-Lithuanian Commonwealth. The Lithuanian nobles boycotted this election, however, and it was left to the Lithuanian ducal council to confirm his election. The commonwealth elected Henry, rather than Habsburg candidates, partly in order to be more agreeable to the Ottoman Empire (a traditional ally of France through the Franco-Ottoman alliance) and strengthen a Polish-Ottoman alliance that was in effect.
A Polish delegation went to La Rochelle to meet with Henry, who was leading the Siege of La Rochelle. Henry left the siege following their visit. In Paris, on 10 September, the Polish delegation asked Henry to take an oath, at Notre Dame Cathedral, to "respect traditional Polish liberties and the law on religious freedom that had been passed during the interregnum".