This page relies very heavily on Alistair Horne’s Seven Ages of Paris.

There being no heir, close to his death, Henri III identified Henri of Navarre as his successor. The problem this posed was that Henri was Protestant, in a generally Catholic country, and certainly in one where there was a strong anti-Protestant movement. The Catholic League controlled Paris and supported a pretender king (pictured at left*), the cardinal de Bourbon who was to rule as Charles X.

In May Henri had successfully defeated forces of the League, augmented by Spanish troops of Phillip II, at Irvy, a short march from Paris. He approached Paris with a modest force (about 12 – 13,000) troops and was denied entrance to the city. Though his force grew to about 25,000, he was never able to gain entrance to the city, though he successfully isolated it from supplies and military relief. His aim became to force in into submission via starvation.

Although Paris had a supposedly much larger defense force, it was a DEFENSE rather than offense force, and Henri was never seriously challenged by those within, although they were able to keep him out. The risk that Henri faced was from the duc de Mayenne’s forces, recently defeated at Irvy, but reorganizing themselves. Paris was not an easy target. This map by Merian from 1615 shows Paris still a heavily fortified city. The Louvre is on the lower right, against the river. The orientation is from the west generally looking east. You can see one of Henri IV's great legacies, the Pont Neuf at the downstream end of Ile de la  Cite.

In addition to its regular population, Paris was filled with refugees from the countryside seeking shelter in the city. This added a burden on the food supply, and many of these refugees were expelled. By the end of the summer starvation was having its impact, more so than the bombardment from Montmartre. Things looked bleak for the forces of the Catholic League but they rejected the offer of amnesty Henri made, in return for their surrender of the city.

Help arrived in the person of the duc de Parma, who had been fighting in the Netherlands for Phillip of Spain. Together Parma and Mayenne succeeded in raising the siege and Henri was forced to withdraw. These forces never met decisively outside Paris. Supplies entered Paris September 1, ending the four month siege.

While Paris remained outside Henri’s control for the moment, the siege cost 40 – 50,000 lives out of a population of about 220,000. This loss of between 20% and 25% of the population contributed to a souring of Parisian support of the League. A change in Popes also ameliorated the attitude of the church, weakening resistance to a Protestant king. This painting of Henri IV is by Frans Pourbus II.**

Outside Paris Henri still fought (in a desultory manner) but was unsuccessful in gaining the city. He realized the control of Paris was key to a secure hold on the throne. In a letter to his mistress before the siege, he said “I am before Paris where God will assist me. Taking the city, I will finally begin to feel the effects of the Crown.”

What he couldn't do by force, he did by diplomacy. In July 1593, three years after the unsuccessful siege, he gave up Protestantism and sought absolution from the Catholic Church. His statement about this conversion is one of the most famous in the history of Paris: “Paris is worth a mass.”

Sources

Horne, Seven Ages of Paris

Guizot, France

*galleryhistorical figures.com

**http://heraldica.org/topics/france/rois_ic.htm