A baby girl who was born on the day the United States entered World War I would have celebrated her 65th birthday last week. Her grandson who was born on the day President Kennedy ordered the first Special Forces into Vietnam would celebrate his 21st birthday next month. The difference in their world views explains a great deal about why Ronald Reagan is having such difficulty dealing with the "nuclear freeze" movement.

If the grandmother--call her Martha--were at all typical, her father might have gone off to war in France while she was an infant. In her mid-20s, chances are her husband, her brothers or her suitors went off to fight again in Europe or the Pacific. It is not at all improbable that in her middle years, she had younger relatives in Korea and Vietnam.

Her grandson--call him Charles--has never had to think seriously about being conscripted and sent off to a foreign battlefield. The draft ended when he was 10, and every president he can remember has pledged not to send American forces into combat--anywhere.

When Charley came home from his junior year at the state university for Grandma Martha's birthday party, he told her that he and his friends were going to give her the best present possible: they were lifting their voices to demand a halt to the nuclear arms race. That will be wonderful, she says, but let's also remember to keep the peace.

From the perspective of their lives, Martha and the president both think of war--and not just a particular weapon of war--as the affliction of mankind. They remember the false hopes of the 1920s, the belief that the democracies had won "the war to end wars." And they saw that false peace shattered in just over 20 years, because the democracies failed to keep their enemies in military check.

As adults, they participated in the debates at the end of World War II that made America for the first time the guarantor of a military alliance in Europe and Asia, aimed at deterring a third world war. In their eyes, that alliance and its military strategy have been an extraordinary success. They see the nuclear deterrent as the source of the tenuous equilibrium that has been maintained since the start of the atomic age. And they see the nuclear freeze movement as a sentiment--not a policy.

But the movement and what it represents are not, for that reason, to be scorned. The instinct that underlies it is the profound human revulsion against the horror these weapons are designed to inflict.

Were they not so horrible, they would not deter. Because they are horrible, they must be disciplined.

The American people have understood that paradox from the beginning. It is that continual reach for rational controls that has made it tolerable for people to live in a world shadowed by the terrible weapons that enforce our shaky hold on peace.

Reagan himself indicated an understanding of this feeling in his first--and only--speech on arms control last autumn. Unfortunately, he came to office saddled with a position of partisan opposition to SALT II, on grounds--which he has never adequately explained-- that it was disadvantageous to the United States.

Since becoming president, he has embraced-- with even less persuasiveness--the even shakier proposition that the Russians now have such an edge that we must delay strategic arms-control until we "catch up," whatever that may mean.

This debate cannot be left at the level of idiocy where the MX missile (with no launching sites) is competing against the nuclear freeze (with no deterrent strategy). The president has to educate a new generation on the need for the nuclear deterrent, and the need to keep that deterrent under restraint. That is the mission Reagan must accomplish in his U.N. speech in June.