On 10 May 1940 Nazi Germany invaded the Netherlands and the government fled to London to escape the German occupation.

Following the end of World War II Queen Wilhelmina ordered a Recall of Parliament and Smallenbroek was appointment as a Member of the House of Representatives taking the place of Willem Wagenaar, taking office on 20 November 1945 serving as a frontbencher and the de facto Whip.

After the election of 1946 Smallenbroek wasn't reelected and he continued to serve until the end of the parliamentary term on 4 June 1946.

Smallenbroek remained in active in national politics, in August 1967 he was nominated as a Member of the Council of State, taking office on 1 September 1967.

In June 1967 Smallenbroek was diagnosed with terminal cancer[citation needed], he died four months later at the age of 65.