Complex sentence: By the time the conference was held, more than 250,000 Jews had fled Germany and Austria, which had been annexed by Germany in March 1938; but more than 300,000 German and Austrian Jews were still seeking refuge and asylum from oppression.
Split into simpler sentences: By the time the conference was held, more than 250,000 Jews had fled Germany and Austria, which had been annexed by Germany in March 1938. But more than 300,000 German and Austrian Jews were still seeking refuge and asylum from oppression.