President Reagan's tentative decision to stay within the limits of the unratified SALT II treaty included directives that set the stage for more bureaucratic battling late this year on the future of U.S. land-based intercontinental missiles as well as continued U.S. adherence to the pact, according to administration officials.

While deciding to "go the extra mile" for arms control by dismantling two Poseidon missile submarines as required by the second Strategic Arms Limitation Treaty, Reagan authorized a "strategic modernization" study to be the focus of new decision-making at the White House in December on future development of land-based missiles.

Also around December, Reagan will face another SALT II limitation issue when the number of bombers carrying air-launched cruise missiles will bump up against the treaty's limits. To stay within the treaty, Reagan would have to suspend the cruise missile modernization program for the B52H aircraft or dismantle some strategic weapon such as another Poseidon submarine or land-based Minuteman III intercontinental ballistic missiles (ICBMs) to compensate for the additional cruise missiles.

Pentagon and Arms Control and Disarmament Agency officials, who favored breaching the SALT II limits in the recent round of policy-making in response to alleged Soviet arms-control violations, said the just-completed White House discussions make it likely that Reagan will exceed SALT II limits at the end of the year. However, some of the same officials expressed similar sentiments last June -- when Reagan made his previous decision to stay within SALT II limits -- that he would break out of the limits the next time the matter was considered.

State Department officials, who argued against exceeding SALT II limits in the latest round, said Reagan is no more committed to jettisoning the treaty limits now than he was in the past.

"Presidents don't set their feet in concrete in advance of decisions to be made," said a State Department official. He noted the similarity between Reagan's hotly contested decisions last June to dismantle a Poseidon submarine and order a nonbinding study of responses to Soviet violations, and his tentative decisions 10 days ago to dismantle two Poseidon submarines and order a nonbinding study of strategic modernization.

Reagan's "tentative" decisions of recent days have been discussed with members of Congress in confidential briefings and with U.S. allies in Western Europe, Asia and Canada by special arms advisers Paul H. Nitze and Edward L. Rowny. The allies were expected to approve the decision to remain within the SALT II limits. Congress is divided on the issue, with a majority of the Senate and the House favoring continued adherence to SALT II, and a Senate minority including conservative friends of Reagan urging that the treaty limits be ignored.

Reagan's decision on the dismantling was made despite the recommendation of Defense Secretary Caspar W. Weinberger and other key advisers that he put the two submarines on "caretaker status," where they would be unarmed but capable of returning to service as missile-firing subs after a costly overhaul.

Reagan "could finesse a decision to breach the treaty this time," said a congressional defense expert, "because keeping old Poseidons going was a dumb idea from both a cost and military angle."

When the next decision on the treaty comes late this year, "the whole nuclear force structure will be on the line," said a former Pentagon official, "and there will be no easy way out."

The Pentagon had scheduled a December production decision on the controversial Midgetman, a single-warhead mobile ICBM, which had been pushed by Democrats in Congress. It has been under Air Force development since early 1983, but recently came under criticism from key Pentagon civilians including Undersecretaries Fred C. Ikle and Donald Hicks.

Hicks, who heads research and engineering, has questioned the utility of the single-warhead weapon and requested that in preparing for the December decision, the Air Force also consider both a multi-warhead version of Midgetman and a larger, new multi-warhead mobile ICBM, which the Pentagon has nicknamed NICM (pronounced "nickem").

December also will be when the next potential U.S. SALT II violation will arise from modifications of B52H strategic bombers to carry up to 20 air-launched cruise missiles. Under the treaty, the United States is allowed 120 such bombers, a figure it will reach in the next few months. If the U.S. total goes above that number, each new bomber will count against the overall SALT II limit of 1,200 multi-warhead missiles.

Weinberger announced earlier this year that the Air Force intended to convert all its B52Hs to cruise missile carriers, a program that would put the U.S. total of such aircraft at 195, 75 bombers above the limit. If that program proceeds, an equal number of Poseidon or Minuteman III multi-warhead missiles would have to be dismantled for the United States to remain within the treaty.

The Air Force to date has no plan to take down any of its ICBMs, according to Pentagon and congressional sources. The Navy has told Congress it would be unhappy about dismantling its weapons to meet an Air Force problem.

Conservative legislators, who urged Reagan to break out of the treaty, were told by White House aides after Reagan's tentative decision that the president did not consider the idea of making submarines inoperable a viable military response to what the White House had charged were Soviet treaty violations.

They have been told that next time, the air-launched cruise missiles will give him the type of weapons decision he wants.

Pro-SALT legislators, on the other hand, have been told that if Congress continues to refuse funding for additional MX missiles, the president will be forced to breach the treaty with additional cruise missile-carrying B52s and to seek a multi-warhead Midgetman, according to one key congressional aide. "They are playing hardball," he said.

One Reagan-allied Republican on the Senate Armed Services Committee, who has been briefed on the president's plans, said White House aides see the cruise missile decision as "the trigger mechanism" for December. "They feel they have a hammer hanging over both the Soviets and the Congress." he said.