In the 1920s, Yahya had expanded his power to the north into Tihamah and 'Asir, but he collided with the rising influence of the Saudi king of Nejd and Hejaz, Abdul Aziz ibn Sa'ud. In the early 1930s, Saudi forces retook much of these gains before withdrawing from some of the area, including the southern Tihamah city of Al Hudaydah. The present-day boundary with Saudi Arabia was established by 20 May 1934 Treaty of Taif, following the Saudi–Yemeni War in 1934. Yahya's non-recognition of his kingdom's southern boundary with the British Aden Protectorate (later the People's Democratic Republic of Yemen) that had been negotiated by his Ottoman predecessors resulted in occasional clashes with the British.