Donald Trump, America’s previous president, fuelled those concerns with his isolationist rhetoric and his view of alliances as expensive favours to free-riding foreigners. 
黑体部分是介词短语，作定语修饰 alliances，“作为......的结盟”。如果 view 是动词，那么alliances 则是宾语，as expensive favours to free-riding foreigners 是宾补，即：He viewed alliances as expensive favours to free-riding foreigners. 是动词 view 接复合宾语结构。

【翻译】美国前总统唐纳德•特朗普（用他）的孤立主义言论以及他认为结盟是对搭便车的外国人的昂贵优惠待遇，加剧了这些担忧。