In 1864 and early 1865 Paraguayan forces under the orders of Francisco Solano López seized Brazilian and Argentine shipping and invaded the provinces of the Mato Grosso and Rio Grande do Sul (Brazil) and Corrientes (Argentina).  On 1 May 1865 Brazil, Argentina and Uruguay signed the Treaty of the Triple Alliance against Paraguay.  By Article XVIII of the Treaty its provisions were to be kept secret until its "principal object" should be obtained.
One of its provisions concerned the acquisition by Argentina of large tracts of territory then in dispute between it and Paraguay.  Lettsom was not satisfied about this and surreptitiously obtained a copy of the Treaty from the Uruguayan diplomat Dr Carlos de Castro.  He forwarded it to London and the British government ordered it to be translated into English and published to Parliament.  When the text became available in South America there was outrage in several quarters, some because of the Treaty's content, others because it had been published at all.
Lettsom has been cited as an exemplar of the nuance with which a substantial part of the British diplomatic corps saw the Paraguayan War.