Complex sentence: Beneš was a member of the Czechoslovak National Social Party (until 1925 called the Czechoslovak Social Party) and a strong Czechoslovakist--he did not consider Slovaks and Czechs to be separate ethnicities.
Split into simpler sentences: Beneš was a member of the Czechoslovak National Social Party, called the Czechoslovak Social Party until 1925. A strong Czechoslovakist, he did not consider Slovaks and Czechs to be separate ethnicities.