WHEN Sir Winston Churchill, an impecunious gentleman from the west of England, learned that his promising soldier son John wished to marry Miss Sarah Jennings he was enraged. The Jennings family was respectable enough and the girl was a beauty, but she was poor and Sir Winston had already selected a rich heiress as a suitable bride for his exceptionally attractive son. John Churchill was the handsomest man of his day and had already charmed some of the most powerful women at the court of Charles II. "Of all the men I ever knew (and I knew him extremely well)," wrote Lord Chesterfield many years later, "the late Duke of Marlborough possessed the graces in the highest degree, not to say engrossed them . . . His figure was beautiful but his manner was irresistible by either man or woman."

The penniless young officer won his way and Virginia Cowles has written the fascinating story of his marriage. As she tells us, "It was not only a love match, a rare occurrence in the 17th century, but it proved to be a great political partnership." The climb to the top started when Colonel and Mrs. Churchill accompanied the Duke of York, brother to King Charles and heir to the throne, into a four- year exile. At the same time Sarah became an intimate friend of the duke's daughter, Princess Anne, a pathetic and lonely girl who had few close companions. She was married to Prince George of Denmark, a dull lout of whom Charles II was to say in a famous remark: "I have tried him drunk, and I have tried him sober and there is nothing in him." He was to do his duty by providing the princess with a child nearly every year, but all of them were fated to die either in infancy or in early childhood. Anne's life was an empty one until the radiant 22-year-old Sarah danced into it. When in 1685 James II succeeded his brother, both Churchills were given important positions and lucrative stipends; he as Lord Churchill, advisor to the king, she as lady of the bedchamber and favorite of the princess who stood next but one to the throne.

Virginia Cowles faces up to the probability of a lesbian relationship between Princess Anne and Sarah which earlier biographers have been inclined to gloss over. However, she considers it impossible to judge the true relationship as Sarah insisted that the princess should destroy her letters. She gives us Anne's cry: "In obedience, after having read & kissed your dear kind letter over and over I burnt it much against my will & I do assure you you need never be in pain about your letters for I take such care of them 'tis not possible that any accident will happen that they should be seen by anybody."

It is curious that Sarah should have been so cautious about her letters, for in her old age she wrote with candid arrogance about her domination of the plain, shy girl who was consumed with adoration for her. In that account she refers to herself in the third person as "the Dutchess." "The Dutchess . . . began to employ all her wit and all her vivacity and almost all her time to divert and entertain and serve the Princess and to fix that favour which now one might easily observe to be increasing toward her every day. The favour quickly became a passion; and a passion which obsessed the hearts of the Princess too much to be hid." Sarah's narrative continues, describing that the princess and duchess were shut up together for many hours daily. "To see the Dutchess was a constant joy; to part with her for never so short a time a constant uneasiness; as the Princess' own expressions were. This worked even to the jealousy of a lover. She used to say that she desired to possess her wholly; and could hardly bear that she should ever escape from this confinement into other Company."

The passion was entirely one-sided. Sarah was as impatient as she was high-spirited, and she found her attendance on the princesss a tedious duty which she evaded as often as she could in order to rejoin her beloved John. When the Glorious Revolution of 1688 took place, the Churchills threw in their lot with William and Mary and were rewarded with a golden harvest of riches and the titles of earl and countess of Marlborough. As lieutenant- general of the British forces Marlborough distinguished himself in Ireland and France in the last years of the century, but his true military genius was given its great opportunity after the deaths of William and Mary. By 1702 Queen Anne, Mary's sister had succeeded to the throne and the Marlboroughs' hour had come.

The new queen remained as dependent on her favorite as she had as princess, and Lord Marlborough was appointed as commander-in-chief, facing a hostile Europe dominated by Louis XIV of France. To oppose the enemy he marched the British army from the North Sea to the Danube in a superb and dangerous thrust, and having formed a coalition of allies to assist him in his gamble he fought on from success to success. Blenheim, Ramillies and Oudernarde were his great victories, celebrated to this day in military annals.

In England a grateful queen and nation awarded him a dukedom and the gift of an estate on which was to be erected a magnificent palace, built by the famous architect Sir John Vanbrugh and named Blenheim in honor of Marlborough's great triumph.

During the first decade of the new century the captain- general could rarely return home from his continental campaigns, but his letters to his wife demonstrated his undiminished love for her. Sarah destroyed her letters to him, but it is certain that she never faltered in her devotion to her husband. With others she could be hard and ruthless; her children knew her as a cold and humorless mother. While she had had immense charm as a younger woman, in maturity the less attractive sides of her nature emerged sharply. Vain and self-centered, she could be an intransigent enemy. As the years went on, the queen grew to prefer the easier company of a cheerful, kindly woman called Abby Masham, and this roused Sarah to vicious fury. Virginia Cowles deals well with the emotional-political drama that ensued, and at the end of the book we are as fascinated as we are repelled by the strong woman who was loved by the great Duke of Marlborough. Cowles' research is thorough and as an experienced war correspondent herself she takes us through the intricacies of Marlborough's wars with competent ease. The maps and illustrations are well done and the book is written in a clear and readable style.