Complex sentence: The Great Game is proposed to have happened between the time of the Russian victory over Iran in the Russo-Persian War (1804--13) to the Joint Pamirs Boundary Commission agreement of 1895 when the border between Afghanistan and the Russian empire was defined.
Split into simpler sentences: The Great Game is proposed to have commenced at the time of the Russian victory over Iran in the Russo-Persian War (1804--13). It concluded with the Joint Pamirs Boundary Commission agreement of 1895, when the border between Afghanistan and the Russian empire was defined.