Complex sentence: At first advocating pacificism and appeasement, he later reversed his position; by 1938 he became a strong critic of Neville Chamberlain's attempts to appease Adolf Hitler and Benito Mussolini.
Split into simpler sentences: At first advocating pacificism and appeasement, he later reversed his position. By 1938, he became a strong critic of Neville Chamberlain's attempts to appease Adolf Hitler and Benito Mussolini.