Rafe de Crespigny, from Northern Frontier wrote:The northwest thus reverted to a backwater in the affairs of the empire, though Ma Teng and Han Sui did establish a form of alliance with Cao Cao, which secured their benevolent neutrality while Cao Cao was most heavily engaged in his campaigns across the North China Plain and south towards the Yangtse.49 However, after his defeat in 208 by the forces of Sun Quan and Liu Bei at the battle of Red Cliffs on the Yangtse, Cao Cao paid increasing attention to the northwest. About the same time, Ma Teng and Han Sui became involved in a feud with their followers. Ma Teng was defeated and came east from Hanyang into Youfufeng.50 In the following year Ma Teng himself was persuaded to attend the capital, where he remained a virtual prisoner while his son Ma Chao succeeded to his command.51
In 211, however, when Cao Cao's commander Zhong Yao attempted to move into the Wei valley, the incursion produced a defensive alliance of all the leaders in the northwest, and an army of ten divisions, headed by Ma Chao and Han Sui, came to oppose Cao Cao's forces at the pass of Tongguan, south of the great bend of the Yellow River.52 Cao Cao himself came to take command of the operations. Instead of pressing a direct attack against the west, he maintained his forces in defence lines at Tongguan while a detachment was sent north to establish a bridgehead across the Yellow River at Puban.53 Then he took his main army to follow them, crossed the Yellow River north into Shaanxi and marched south with the river guarding his flank. Hving turned the line of the enemy's major defensive position, he crossed the mouth of the Wei and defeated the allied army in pitched battle near Huayin, on the plain south of the river.55 Several of the petty chieftains were killed, and Cao Cao's forces occupied the region of Changan and Anding commandery.56
49 SGZ 13, 392-93, being the Biography of Zhong Yao, who was Cao Cao's chief agent in the northwest.
50 SGZ 36 (Shu 6), 945, commentary quoting the Dian lue of Yu Huan.
51 HHS 72/62, 2343; SGZ 15, 472. Ma Teng was at first treated with honour and given a ministerial post, and the title of Marquis of Huaili, but he was executed in 212, after the rebellion of his son Ma Chao.
52 Tongguan was the chief pass and defensive position on the direct road into the Land within the passes.
This campaign is described in SGZ 1, 34-35, and SGZ 36 (Shu 6), 945-46, with commentary quoting the Shanyang gong zaiji of Yue Zi and Dian lue of Yu Huan. The parallel text in ZZTJ 66, 2106-08 is translated by de Crespigny, The last of the Han, 282-285.
53 The Puban Crossing was a crossing of the southward course of the Yellow River now west of Yongji in Shanxi.
54 At this early stage of his march, as he was retreating from Tongguan and crossing the Yellow River to the north, Cao Cao's forces were hotly pursued by those of Ma Chao. Cao Cao himself took command of the rear guard, and to show his calm and confidence in the defence, he sat upon huchuang, a "barbarian bed", evidently a light and portable piece of camp furniture, and the fore-runner of the Chinese chair. See Fitzgerald, Barbarian Beds.
55 As shown on the map, Cao Cao's manoeuvres before the decisive battle are an impressive example of the oblique approach, reminiscent, on a smaller scale, of his campaing leading to the Battle of White Wolf Mountain against the Wuhuan in 207. Besides the major sources indicated in note 52 above, other references to this campaign are cited by Haloun, "Liang-chou Rebellion", note 54.
56 Yang Q@iu, the warlord of Anding, had been one of the allies against Cao Cao. After the defeat, he fled back to his base, but he was besieged there and surrendered. Cao Cao accepted his submission and confirmed him in his local power. In later years Yang Qiu was promoted to high court and military rank, enfeoffed as a marquis, and he died of old age. (SGZ 1, 36 and commentary quoting the Wei lue of Yu Huan.)
So, as you can see, Cao Cao's forces made the first move. So the question is, was he right to defend his independence at his father's expense?
I couldn't find this part in Zhong Yao's biography. In case I did not see Rafe's references or you did not copy correctly, can you please check if Rafe put any references for this section? (52 is only an explanation of Tongguan). Of cos, I'll keep a look out in SGZ too. 
