Complex sentence: During the Peninsular War, the Miquelets successfully harassed the French occupiers in the mountains of Catalonia, sometimes even participating in operations in large bodies, such as in the Siege of Gerona (1809) and other operations around Girona in 1808 and 1809.
Split into simpler sentences: During the Peninsular War the Miquelet were exceedingly successful in harassing the French occupiers in the mountains of Catalonia. Sometimes they even attempted operations in large bodies, as in the operations around Girona in 1808 and 1809.