FORWARD presence - deterring aggressors by massing forces in potential conflict areas - is a constant challenge for a budget-conscious military. Heavy armor and battlefield weapons and other elements of large-scale deployments can only move by sea, and sea transit is slow. A major power needs far-flung bases, or quick transport.
This simple fact underlies the recent call by Adm. William Owens, vice chairman of the Joint Chiefs of Staff, to ratify the new Law of the Sea Convention. Owens's challenge to the Senate to set aside this season's isolationist-internationalist debating and ratify the treaty was not an exercise in sentiment or naive multilateralism. The navigation rights guaranteed by the treaty will strengthen US defense capabilities.
The treaty sets out a right of ''transit navigation'' - permitting military vessels, such as carrier battle groups and protective fighter escorts, to take a straight line course even within 12 miles of shore.
One litmus test for the treaty is in a hypothetical deployment from the US naval base at Yokosuka, Japan, to the Persian Gulf- spigot for much of Europe's and Japan's oil. Transit navigation rights permit a battle group to snake its way through the island groupings of Southeast Asia - the shortest course to the Middle East.
Without transit rights, a battle group would have to proceed around the southern pacific coast of Australia, adding 15 days and fuel costs of $2.9 million for six ships.
Transit rights also permit strategically important US nuclear submarines to remain submerged while passing though straits. Many essential sea lanes pass close to powers not always aligned to US interests. Even friendly countries may be reluctant to associate themselves with US military actions. Under the treaty one can avoid questions of political delicacy in a crisis.
Seventy-five countries have ratified the treaty to date, including Germany, Italy and Australia. The United Kingdom and Japan are expected to ratify by early 1996. The triumph of the treaty is that US negotiators managed to win such favorable navigation rights, at a time when no other naval power gains as much benefit by them.
Transit rights are also essential to protection of US interests in the Mediterranean. The 1986 US raid on Libya, designed to discourage Muammar Qaddafi from continued terrorism, could be carried out only through overflight of the Strait of Gibraltar. The treaty lays to rest objections to transit passage sometimes made by states such as Morocco and Spain, which abut the Strait of Gibraltar - as well as Indonesia, Malaysia, and the Philippines. It thus minimizes the political cost of crisis response to protect long-term interests.
Early US objections to the Law of the Sea Convention concerned deep seabed mining. The initial text appeared to make the deep seabed an international resource, regulating extraction by independent entrepreneurs. But two major changes have since occurred. First, the economics of deep sea mining have turned unattractive for the indefinite future. Manganese nodules are not about to be scooped from the ocean floor by anyone's seaborne backhoe.
Second, US and European objections were largely met by an amending document, informally called the ''Boat Paper'' - negotiated at the insistence of the industrial nations.
Will the US ratify? It is possible that Foreign Relations Committee chairman Jesse Helms may allow a vote as a courtesy to longtime treaty sponsor Claiborne Pell, who retires next year.
A strong argument for quick action is that the US would maintain its influence in the formation of agencies to implement the convention. A law of the sea tribunal has been formed in Hamburg, and will take most of the law of the sea business away from the International Court of Justice. If the US stays out of the game, it can't expect to win.