The United States, in keeping with President Reagan's election promise, has just begun the largest and most expensive peace-time military build-up in its history. The Pentagon is embarking on a one and a half trillion dollar spending spree over the next five years. Ageing battleships are being taken out of mothballs to be re-equipped with the very latest weapons. There will be new nuclear missiles, and a ' gunboat diplomacy' force of paratroopers ready to fight at a moment's notice, if necessary, in the deserts of the Gulf. But what lies behind these military developments? Tom Mangold looks at the new weapons, at the men trained to use them and their leaders