Through military aid and exercises, the United States is making clear its growing reliance on Africa for defense of its vital oil interests in the Middle East.
The aid is going to Chad, in part, to thwart spreading Libyan influence that might wash into and destabilize Sudan, which lies across the Red Sea from oil-rich Saudi Arabia, according to specialists in Africa and in the United States.
The US recently increased military aid to Chad to $25 million, including antiaircraft missiles and advisers to instruct Chadian soldiers in their use. Two AWACS electronic surveillance planes have been sent to monitor air activity, along with F-15 jets and other reconnaissance equipment. The French have pledged substantial military hardware, but have so far resisted Chadian pleas for air and troop support to counter Libyan intervention. (See story, Page 8.)
For some time the US has been planning to hold military exercises starting this week with three African nations - Egypt, Sudan, and Somalia - as well as with Oman.
All four countries are strategically placed to help the US protect Middle East oil fields. (Oman is an oil producer in its own right.)
The exercises are a reminder of the fundamental change in American foreign policy that took place in 1980. The fall of the Shah of Iran a year earlier and the Soviet invasion of Afghanistan prompted then-President Jimmy Carter to commit the US to militarily protecting its oil interests in the Gulf region.
Over the past three years, that strategy has solidified, with the development of a Rapid Deployment Force ready to quickly intercept trouble anywhere, but particularly in the Middle East, and increased US military preparedness in the Indian Ocean-Persian Gulf region.
As recently as 1977, the United States was discussing with the Soviet Union the idea of completely demilitarizing the Indian Ocean. The ''zone of peace'' idea is now moribund.
The US exercises in Somalia and Oman, to begin early this month and end by the beginning of September, are illustrative of one major component in the American strategy of increasing its presence in the Indian Ocean, and thereby protecting vital oil interests.
The United States has signed ''access'' agreements with Somalia, Oman, and Kenya that give it the right to use local air and naval bases. The precise terms of these ''access'' agreements are not known, as they are classified.
Oman's facilities are probably the most important to the US, because the sultanate sits near the mouth of the Gulf. The US has access to the airfield on the island of Masirah, which military analysts consider a good staging base because of its location and its sparse population. The Oman agreement was signed in 1980.
Although farther from the Gulf, Somalia is valued by US military planners because it is still close enough to put long-range bombers within range of almost any potential trouble spot in the region.
Also, Somalia boasts good facilities. The former Soviet airbase at Berbera has a large runway capable of handling much traffic. Also, the harbor at Berbera has space for repairing ships and has sufficient warehousing ability for military storage. The agreement with Somalia was also signed in 1980.
The US access agreement with Kenya may have the least strategic value because of the country's distance from the Middle East. But Kenya's relative political stability (although shaken by a coup bid last year) and pro-West leanings make it a valuable ally in the region. The ''access'' agreement with Kenya, signed in 1980, revolves around the use of Mombasa. The port is now used mainly for shore leave for US sailors. But it has potential as a resupply depot and US funds have been used to upgrade the local airport for such a possible future use. In addition to the ''access'' agreements, the United States has beefed up its presence in the Indian Ocean by upgrading Diego Garcia, its only major base in the region, and by increasing the presence of naval forces in the area.
The Diego Garcia atoll was transferred from Mauritian administration by the British in 1965 and then leased to the US for 50 years. Used initially as a communications facility, its role has steadily increased in importance.
US Navy officials now speak of it as a ''major logistical support base.'' It is a large storage facility for oil and munitions and has been dredged to accommodate America's largest aircraft carriers. Diego Garcia appears to be designed so that in the event of trouble in the Middle East it would provide supplies for the Rapid Deployment Force.
Rounding out increased US access to military installations is an increased American naval presence in the Indian Ocean. In the 1970s only a few destroyers operated in the region. But during 1980 and 1981, there were on average 24 warfare and supply ships in the region at any given time, according to military analysts. However, the US naval presence is reported to have been reduced since late 1981. US AID TO CHAD US sends $25 million in military aid, two AWACS planes, and several advisors to help Chad government fight rebels and Libyan forces in the north. JT. MILITARY MANEUVERS US Rapid Deployment Force holds exercises Aug. 10 through early September with Somalia, Egypt, Sudan, and Oman. US ACCESS TO MILITARY BASES US has pacts with Somalia, Oman, and Kenya giving rights to local air and naval bases. Terms of agreements are not known.