中原大戰勝負底定之際，殘餘的西北軍，分成兩股，一股由鄭州北渡黃河，退往豫北，另一股由洛陽往西撤退；目的地都是陝甘根據地。中央深恐重蹈1929年西北軍退回陝甘再次稱兵之覆轍，採用剿撫兼施策略，一方面派大軍追擊，另一方面則用各種方式招降。當宋哲元退至潼關時，後路已被截斷，西退陝甘根據地已不可能，又不願接受中央招降，只得倉促率部退入晉南，展開他客居晉南的艱辛歲月。其實宋哲元並非無意求和，他提出的條件是中央先停止軍事行動，並派西北軍正式代表接洽；中央則要他先通電服從中央，並派個人代表商洽善後辦法。對宋哲元而言，中央的條伴無異是要他簽「城下之盟」，因此他只有選擇相應不理一途。宋哲元退入山西後，有鑒於內戰之慘烈，此時他以「本軍之槍口誓不向內」明志，也就是說，此後他將揚棄為人做內戰的工具，改走對抗帝國主義侵略之路線。經由幾位得力智囊的積極奔走，宋哲元親赴天津接洽改編，以及張自忠關鍵時刻的擁護，終獲中央任命為陸軍第三軍軍長。稍後，中央整編全國陸軍，將陸軍第三軍改為陸軍第二十九軍。入晉西北軍初步整編為兩軍十二師，宋哲元獲任軍長後，奉命將其縮編為一軍兩師四旅制。由於人數眾多，員額有限，宋哲元參考各部實力與資歷，縮編成一軍兩師六旅制，明顯的編制與中央之規定不符，誠如宋哲元所說，其目的在「延統系而維團體」。也就是因為宋哲元這段時問的「苦心毅力」，二十九軍才得以在1933年進入察哈爾後迅速擴編，並於抵禦外侮上綻放光芒，留名青史。

By the end of the Chinese Civil War, the remaining Northwest Army was divided into two parts, one crossed the Yellow River by Zhengzhou and arrived at Honan, and the other went via Loyang to Shaanxi and Gansu. In order not to commit the same error of leading the army to fight in the two provinces in 1929, the central government adopted the strategy of chasing the bandits and pacifying them at the same time. On the one hand, the central government kept on chasing and hunting the bandits, but on the other hand the government also tried many measures to call for their surrender. When Sung Zhe-yuan retreated to Tungkwaun, there was no way to turn back and it was impossible to stay at the military bases at Shaanxi and Gansu. As he would not surrender, he had no choice but to retreat to southern Shanxi and stayed there for some time.As a matter of fact, Sung did not seriously intend to resist the central government's call for reconciliation. He requested the central government to first stop military operations and then to send representatives from the Northwest Army for negotiation. But the central government insisted that he must send a telegram to express the goodwill first and then send representatives for negotiation later. To Sung, the central government's command appeared to be "a treaty of alliance under the city wall," an act that gave him big disgrace. Therefore, he could not but choose to ignore it.After having witnessed the disastrous results of the civil war, Sung declared that he would not open fire to "his fellow countrymen." That is, he would not serve as a "tool" to kill his own people. Sung chose to fight against the invasions of foreign imperial states. With the help of a good think-tank, Sung went to Tientsin in person to discuss re-organization. And with the timely support and help from Chang Zi-zhung, Sung was finally appointed by the central government as the commander of the Third Division of the Northeast Defense Army. Shortly afterwards, the central government started to reorganize the national army and the Third Army at the Northeast of China was renamed as 29(superscript th) Army.The Northwest army at Shaanxi was initially reorganized and divided into two armies with 12 divisions, but after Sung took commandership and after a careful consideration of its size and resources, it was organized into just one army with two divisions and four brigades. This decision was in disagreement with the central government's principles. But as Sung claimed, it aimed at "maintaining the system and uniting the military body." It was due to his act of "taking great pains and keeping perseverance" that the 29(superscript th) Army was able to enter Chahar (now part of Inner Mongolia) in 1933 and succeeded in protecting the nation against foreign invasions with glory and brilliance.