Complex sentence: At the start of World War I, he was giving concerts in Austria with José Iturbi when he was accused of espionage and jailed for a short time, until the intercession of the Spanish king Alfonso XIII secured his release.
Split into simpler sentences: At the start of World War I, he was giving concerts in Austria with José Iturbi when he was accused of espionage and jailed for a short time. The intercession of the Spanish king Alfonso XIII secured his release.