Complex sentence: On several occasions the Dutch vetoed Marlborough's attempts to engage his opponents in the field, but losing a battle in the Low Countries could have potentially fatal consequences to the security of the Republic, and Marlborough himself was a relatively inexperienced foreign general.
Split into simpler sentences: On several occasions the Dutch vetoed Marlborough's attempts to engage his opponents in the field; but they naturally had concerns. Losing a battle in this theatre would have potentially fatal consequences to the security of their own homeland, and Marlborough himself was a relatively unknown, inexperienced foreign general.