Another Churchill calls for a stronger US; Defending the West, by Winston Churchill II. Westport, Conn.: Arlington House. 256 pp. $14.95.

The resemblance between Winston Churchill II and his renowned grandfather extends beyond facial features to action as well. Long before becoming prime minister, Sir Winston was a fiercely independent member of Parliament, who doubled as roving correspondent in troubled corners of the world. Winston II is also a Conservative MP, not always dazzled by his party's policies, and his own journalistic drive to see things for himself has taken him to remote parts of the world.

A more startling similarity is the strong echo in young Winston's new book of the kind of warning the senior Winston sounded back in the 1930s. The arch villain now is the Soviet Union rather than Nazi Germany, but the theme remains much the same: the unpreparedness of the free nations of the West to face the growing threat.

Young Churchill's premise here is that the United States, on which the free world must depend for leadership, has lost its supremacy. ''The Balance of Terror, under which mankind lives today . . . is now in danger of getting out of balance,'' he writes. Soon, unless a new course is set, he warns, the West will not be able to forestall the global ambitions of the USSR.

Though the warning is clear, the book goes further, to survey the changes from the Allied victory in World War II to the current period of what he sees as mortal peril.

The journey, in his retelling, recounts the days of clear-cut NATO supremacy during the '50s and the watershed moment during the Cuban Missile Crisis of 1962 , after which the US, ''having confirmed her position of commanding strength, was to allow her power, self-confidence and resolve to become completely undermined.''

The decline of the West began with the tragedy of Vietnam and continued during the decade of detente - years during which American naval strength dropped by half and the US adopted a doctrine of deliberate vulnerability.

''The dawn of the eighties,'' Mr. Churchill writes, ''ushered in an era in which every one of NATO's offsetting advantages in the field of nuclear weapons - at strategic, Euro strategic and battlefield level -- has been lost.'' Because of the ''surge and momentum'' of the Soviet arms program, America could be the loser in an ultimate exchange.

Mr. Churchill's own adventurous reporting contributes to his estimate of the Soviet intent. He sees the Afghan invasion as ''a deliberate act of policy, forming part of its grand design to encircle, subvert and eventually dominate the key areas of Arabia and the Persian Gulf.'' Just as the Near East is the treasure house of oil, so Africa is the treasure house of mineral wealth. Soviet ground rules for an African takeover were chillingly revealed to the roving reporter by President Barre of Somalia shortly after that courageous statesman had expelled some 3,500 Soviet ''military advisers.'' The gist of the 3 1/2-hour interview is one of the best things in the book.

What can the West do, as the tocsin sounds again?

First of all, says Mr. Churchill, the public must be told the truth. Western statesmen must stop lending their ''tacit connivance'' to the Soviets by their unwillingness to reveal facts that would hurt them at the polls.

If American conventional and nuclear forces can be restored to balance with Soviet forces, Churchill argues, the West may, at some future time, be able to negotiate again from strength, not weakness. For arms control is now the last best hope on earth.

During the early days of World War II, Prime Minister Churchill used an apt quotation (from Arthur Hugh Clough): ''Westward, look, the land is bright.'' Now his grandson, eloquent and farseeing, too, is sending a similar message. There is hope, and promise, but only if there is full awareness of the danger -- and a renewal of American strength.