By the later 1960s , Sen -- with increasing experience of guerrilla warfare -- had built up a reputation as a talented field commander .
By the later 1960s , Sen -- with increasing experience of guerrilla warfare -- had risen to a senior rank within the Party .
By the later 1960s , Sen -- with increasing experience of guerrilla warfare -- had built up a reputation as a talented field commander , and had risen to a senior rank within the Party .