Sentence:  Cuban S. S. R. : Whatever may have been the setbacks resulting from the unsuccessful attempt of the Cuban rebels to establish a beachhead on the Castro held <WSD>mainland</WSD> last week , there was at least one positive benefit , and that was the clear-cut revelation to the whole world of the complete conversion of Cuba into a Russian dominated military base . 
Meaning of the word marked with <WSD> tags:  the main land mass of a country or continent; as distinguished from an island or peninsula