Complex sentence: The Russians were afraid of the Qing forces, thousands of whom were armed with modern weapons and trained by European officers, because the Russian forces near the Qing Empire's border were under-manned and under-equipped, so they agreed to negotiate.
Split into simpler sentences: The Russians were afraid of the Qing forces, thousands of whom were armed with modern weapons and trained by European officers. Because the Russian forces near the Qing Empire's border were under-manned and under-equipped, they agreed to negotiate.