Complex sentence: Méry undertook a long expedition in 1892 organized by Georges Rolland and M. Tharel with the purpose of renewing relations with the eastern Tuaregs of the Kel Ajjer confederation that had been established by Henri Duveyrier and the Polignac Treaty of 1862, which had been allowed to lapse.
Split into simpler sentences: Méry undertook a long expedition in 1892 organized by Georges Rolland and M. Tharel with the purpose of renewing relations with the eastern Tuaregs of the Kel Ajjer confederation. The Tuaregs had made an agreement with Henri Duveyrier (1840--1892), formalized by the Polignac Treaty of 1862, which had been allowed to lapse.