Secret Letters Left Unbroken by John Wallis Solved After 400 Years

In 2020, letters in cipher from Louvois, Louis XIV's Secretary of War, to Count Lauzun (1690) were deciphered by Norbert Biermann by finding the key in the archives.

The present article is an attempt to contextualize the content of the letters.

Historical Background

The letters were written shortly before the Battle of the Boyne on 1 July 1690 (OS), in which William III, the Prince of Orange crowned King of England by the "Glorious Revolution" of 1688-1689, defeated the counterattack of the deposed James II.

Count Lauzun, the recipient of the letter, had joined James with a reinforcement of several thousand men provided by France. Louis XIV could not allow England to be under Dutch William, not only because the Prince of Orange was Protestant, but also because he had been a leading figure in opposition to Louis XIV's expansionism since he became stadtholder in 1672. By assisting James II in Ireland, Louis XIV wanted to tie William's hands, while the war in Germany he started in the fall of 1688 appeared to be a long war (the Nine Years' War).

James II held an exile court after the Revolution in the palace of Saint-Germain(-en-Laye) provided by Louis XIV for his use. He landed in Ireland as early as in March 1689. (Ireland had been under the Catholic rule by the Lord Deputy of Ireland, Earl of Tyrconnell, who supported James II.) James entered Dublin and started his rule of Ireland. In the summer, William sent the Duke of Schomberg with ten thousand men to Ireland, but Schomberg lost his men from disease rather than battle.

In January 1690, William declared that he would personally lead a force to Ireland. Although he wanted to embark in March, he could not start the expedition until the parliament closed on 23 May (OS). It was only on 14 June that William landed in Ireland.

James had been reinforced by seven thousand men under Lauzun in March. (Lauzun was more a courtier than a general, and when it came to military matters, he had to rely on Tyrconnell. Appointment of Lauzun to lead the French contingent was largely due to his closeness with James and Mary, Queen Consort, and had been opposed by Tyrconnell.)

When the Williamite army marched south from Belfast, James retreated south from Dundalk (possibly by the advice of Lauzun (see the letter of 25 May below)). The Battle of the Boyne was fought on 1 July (11 July in the Gregorian Calendar (NS) used in Catholic countries including France).

William won and entered Dublin on 6 July. James fled to France. In September, after the siege of Limerick failed, William returned to England, while Lauzun and French soldiers, accompanied by Tyrconnell, left for France. Limerick capitulated in 1691, which marked William's securing of the whole Ireland under his rule.

Louvois' Letters to Lauzun

After the Battle of the Boyne on 1 July 1690 (OS), letters in ciphers were found among Lauzun's possessions. Nottingham, Secretary of State, forwarded them on 13 July to John Wallis, a math professor at Oxford, who served the government by codebreaking from time to time (see another article). Nottingham considered the letters so important that he asked Wallis to return the bearer of the message with his decipherment.

The captured letters included letters from Louvois dated 1, 25, 26, 27 May and 10 June and another from Seignelay dated 22 June.

Wallis deciphered Seignelay's letter, but could not solve Louvois' code.

Louvois to Lauzun, 1 May 1690 (NS)

Louvois starts by forwarding information from England. Although the parliament was cooperative to William (referred to as "the Prince of Orange", because France did not recognize William III), it did not appear that foreign troops were marching to England or Ireland for reinforcement. Only five English regiments serving in Flanders were to be transported to Ireland. But because of confusion in the states of Scotland, William would not be able to start his expedition to Ireland before 15 or 20 May.

Louvois also reports unconfirmed news of the death of the Duke of Lorraine, a great loss to the imperial army.

Louvois to Lauzun, 25 May 1690 (NS)

Louvois forwards words of those who returned from Ireland that James ("the King of England") was extremely tired of being there and the feeling was exacerbated by the reproach made against him for having left England without giving a fight (during the Glorious Revolution). It was feared that James might jump to the first opportunity "without examining the consequences."

Louis XIV expected Lauzun to ensure that James did not give a fight without being necessary and without a great appearance of a good success. Louvois reminds Lauzun of the conversation they had in which Lauzun agreed that by playing for time, the forces of the Prince of Orange would go on diminishing by diseases.

Louvois explicitly bade Lauzun to burn this letter. (Lauzun reported on 26 June that he burned the letter, but considering the fact that this letter is now extant, it appears he only burned the decipherment, as pointed out by Biermann.)

Louvois to Lauzun, 26 May 1690 (NS)

(In exchange for the troops under Lauzun sent to Ireland in March, James provided Irish recruits to France next month (Patrick Clarke de Dromantin (2005), Les réfugiés jacobites dans la France du XVIIIe siècle, p.183 (Google)). This was the birth of the Irish Brigade, which continued as foreign units in the French Royal Army until nationalized in 1791 (Wikipedia).)

Louvois reports Louis XIV's disappointment of the quality of the Irish recruits. The five regiments received had to be reduced to three regiments commanded by Sieurs de Montcashel, Dillon and O'Brien. Moreover, since O'Brien's conduct did not give a good impression, Louis XIV wanted James' "cadet" to be sent instead. (This must refer to the Duke of Berwick, James' bastard son, now aged 19, who was serving in the campaign including the Battle of the Boyne.) Louvois points out that James promised to send as many Irishmen as Louis sent him from France, and suggests that Lauzun induce James to send another 1000 or 1200 men.

From the latest news from England dated 12 May (NS), it did not appear that William could leave for Ireland for a long time, and some even said that he would not go at all because of lack of money.

The rest is a summary of a separate letter sent from Seignelay, Naval Minister, to Lauzun. It appears to be about disturbing communication of the St. George's Channel (between Ireland and England) and land transport of goods between Dublin and ports. It is also pointed out that Limerick (on the west coast of Ireland) is better than Waterford, Kinsale, and Cork, which are nearer to Brittany, but are more dangerous because of passing near the "Sorlingues" (Isles of Scilly).

Louvois also mentions Louis XIV arranged for sending gunpowder and arms from Dunkirk to Scotland to promote an uprising in favor of James. He says that the French king was doing it by the authority of the Queen of England (consort of James in the exile court in France), and spares the details because Lauzun would be informed of it by James.

Louvois to Lauzun, 27 May 1690 (NS)

This letter begins with acknowledging receipt of Lauzun's letters of 3, 5, 12, 16, and 25 April and 1, 4, and 8 May, before which the letter of 1 May acknowledges receipt of letters of 24 and 28 March. On the other hand, Seignelay's letter of 22 June acknowledges receipt of Lauzun's letter of 29 May. (These show varying difficulty in communication across the Channel.)

Louvois conveys Louis XIV's approval of Lauzun's conduct, including his relation with Tyrconnell, who had opposed his appointment. James was pitied for having a subordinate as "Milord Douvre", whose treatment of the French people at Cork was complained of by Lauzun in his letter of 19 April (printed in Ranke).

Louvois then gives advice of making officers responsible for conservation of weaponry and paying only for the number of men who were on foot, and notified (in cleartext) preferments of some officers.

Louvois conveys Louis XIV's approval of the memoire Lauzun presented to James (printed in Ranke).

The rest seems to continue addressing the topics mentioned in Lauzun's letters.

Sending copper to Ireland was difficult because of high price, and it would be better to ship from Hamburg or Sweden, where it was cheap. (James issued base metal coins, called "gun money", to be exchanged for silver when he restored the throne. In 1690, metal was in short supply and he issued smaller coins with the same face value.)

Louis XIV did not consider James was to blame for the confusion in Ireland.

No comments on what happened in Dublin on Good Friday (the Friday before Easter, that is, 24 March in 1690).

Louis XIV approved that Lauzun did not allow the money of the troops to be changed for copper, and the soldiers should not be paid in currency other than that of France.

As noted a few days ago, the news in Lauzun's letter of 8 May about prompt arrival of the Prince of Orange in Ireland is baseless.

As told by Seignelay, dividing the King's fleet (between the St. George's Channel and the English Channel?) as proposed by Lauzun was impractical.

About Lauzun's proposal of sending cavalry, Louvois says that he was sure that Lauzun made the offer only to pacify James because Lauzun knew well that it was impractical.

Lauzun should not employ S. de Tangire.

Louis XIV was dissatisfied with Sieur Boisseleau (Wikipedia). He had no objection to James letting him serve as a major general in his army, but he should not be employed under Lauzun.

Louvois to Lauzun, 10 June 1690 (NS)

Louvois begins with the King's dissatisfaction of the error of the relief sent to Charlemont reported in Lauzun's letter of 20 May. The relief did not come out after entering the town. (Charlemont was under siege by Schomberg. Colonel MacMahon brought food into the fort, but only exacerbated the starvation after consuming the food they brought in, and the fort capitulated in April. (NFB, "Ireland's Wars: Cavan And Charlemont, 1690", Wikipedia))

The King approved Lauzun's putting Irish officers in French regiments to which Irish recruits would be placed.

The King was surprised by a letter of Tyrconnell to the Queen of England, who showed the letter to the King. Tyrconnell considered James should immediately go to England, but Louis XIV did not think it appropriate because James had no foothold and no considerable body to join him. Louis XIV was convinced that by standing on the defensive, illness would ruin the army of the Prince of Orange.

The latest news from England (dated 28 May) indicates the Prince of Orange would leave for Ireland by 20 June.

Seignelay to Lauzun, 22 June 1690 (NS)

Seignelay has already informed Lauzun that Louis XIV resolved to send a squadron of vessels with about fifteen corsairs of St. Malo to St. George's Channel to harass the transport of food and ammunition from England to Ireland. Seignelay dispatched the necessary orders today.

Under the escort of this squadron, Seignelay sent supplies. The King ordered Monsieur Forant to have them escorted to Dublin. But the vessels under his command were mainly intended for St. George's Channel.


John Wallis could decipher Seignelay's code, but he could not solve Louvois' code. As I pointed out in another article, a striking weak point of the French cipher that Wallis could solve is that "low numbers were reserved to code single letters." Biermann's reconstruction of Seignelay's cipher reveals this weak point. In contrast, Louvois' code mixes letters of the alphabet as well as syllables and words all in random order.

Nottingham requested instant codebreaking of these letters. As it turned out, it seems there is little that would have helped the Williamite campaign after the Battle of the Boyne. (The Jacobite strategy of avoiding battle may have been useful before the battle.)

I have to leave to historians to assess whether the deciphered letters include new insight in this part of history.


Norbert Biermann, "'I Suspect Somewhat of Peculiar in His Way of Ciphering'" (Universität der Künste Berlin, DOI). (He also provides a background account at Cipherbrain.)

S. Tomokiyo, "John Wallis and Cryptanalysis" (Cryptiana)

Leopold von Ranke (1868), Englische Geschichte vornehmlich im sechszehnten und siebzehnten Jahrhundert, vii (Google, reprint) Some letters quoted are also available in pdf in Andreas Boldt, "Leopold von Ranke on Irish history and the Irish nation" (researchgate).

John Bergin, "Lauzun, Antonin Nompar de Caumont" (Royal Irish Academy)

©2021 S.Tomokiyo
First posted on 14 March 2021. Last modified on 27 March 2021.
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