As the years went by, the Black Sea Cossacks continued their systematic penetration into the mountainous regions of the Northern Caucasus. Taking an active part in the finale of the Russian conquest of the Northern Caucasus (1817–1864), they settled regions as these were conquered. To aid them, a total of 70,000 additional ex-Zaporozhians from the Bug, Yekaterinoslav, and finally the Azov Cossack Host migrated there in the mid 19th century. All three hosts needed to be removed to vacate space for the colonisation of New Russia, and with the increasing weakness of the Ottoman Empire as well as the formation of independent buffer states in the Balkans, the need for further Cossack defensive presence in New Russia had ended. They migrated to the Kuban in 1860 and merged with the Caucasus Line Cossack Host, which consisted of migrated Don Cossack elements. Finally, in 1864, the Black Sea Cossacks and the Azov Cossacks were united into the Kuban Cossack Host, ninety years after the destruction of the Zaporozhian Sich.