The period between the Mukden Incident in 1931 and the official beginning of the Second Sino-Japanese War in 1937 involved constant regional armed resistance to Japanese advances in Manchuria and North China, and the Nanjing's efforts in stopping further encroachments through diplomatic negotiations. This era was turbulent for the Nationalist government, as it was mired in a civil war with the Chinese Communists and maintained an uneasy truce with remnant warlords, who nominally aligned with Chiang Kai-shek, following the Northern Expedition. This period also saw the Nationalist government's pursuit in modernizing its National Revolutionary Army, through the assistance of Soviet, and later German, advisors.