The decisions were long awaited as crucial to defining the boundaries of presidential power in time of war. Their content was largely a defeat for the administration.
The nine justices ruled narrowly that Congress gave President Bush the power to hold Yaser Hamdi, an American citizen, without charges or trial as an "enemy combatant," but said the detainee can challenge his treatment in court.
The court also ruled that people seized as potential terrorists may file suit in American courts to challenge their captivity. The ruling affects detainees in Guantanamo Bay, Cuba.
The vote in both cases was 6-3.
Steven R. Shapiro, legal director of the ACLU, called the rulings "a strong repudiation of the administration's argument that its actions in the war on terrorism are beyond the rule of law and unreviewable by American courts."
The justices said they would not rule in a third case concerning the war on terrorism — that of "enemy combatant" Jose Padilla.
The administration had fought any suggestion that Hamdi or another U.S.-born terrorism suspect could go to court, saying that such a legal fight posed a threat to the president's power to wage war as he sees fit.
"We have no reason to doubt that courts, faced with these sensitive matters, will pay proper heed both to the matters of national security that might arise in an individual case and to the constitutional limitations safeguarding essential liberties that remain vibrant even in times of security concerns," Justice Sandra Day O'Connor wrote for the court.
O'Connor said that Hamdi "unquestionably has the right to access to counsel."
The court threw out a lower court ruling that supported the government's position fully, and Hamdi's case now returns to a lower court.
O'Connor said the court has "made clear that a state of war is not a blank check for the president when it comes to the rights of the nation's citizens."
Two other justices, David H. Souter and Ruth Bader Ginsburg, would have gone further and declared Hamdi's detention improper but joined in O'Connor's opinion.
"Even though the Court recognized the president's power to detain a U.S. citizen as an enemy combatant, the ruling is a significant defeat for the administration because the justices recognized that this power has important limitations," said CBS News legal analyst Andrew Cohen.
However, the ruling may allow the Bush administration to grant Hamdi the required court hearing in a military tribunal rather than a civilian court.
Hamdi, captured on the battlefield in Afghanistan, and Padilla, arrested in Chicago on suspicion of plotting a terrorist act, have been held in solitary confinement for most of the past two years. A lawyer who has never met Hamdi appealed his case to the Supreme Court.
The Court ruled that Padilla improperly named Defense Secretary Donald H. Rumsfeld instead of the much lower-level military officer in charge of the Navy brig in South Carolina where Padilla has been held for more than two years. Padilla must re-file a lawsuit challenging his detention in a lower court — which, Cohen says, "guarantees that the Padilla case will be around for years to come."
The Bush administration contends that as "enemy combatants," the men are not entitled to the usual rights of prisoners of war set out in the Geneva Conventions. Enemy combatants are also outside the constitutional protections for ordinary criminal suspects, the government has claimed.
The administration argued that the president alone has authority to order their detention, and that courts have no business second-guessing that decision.
Congress voted shortly after the Sept. 11 attacks to give the president significant authority to pursue terrorists, but Hamdi's lawyers said that authority did not extend to the indefinite detention of an American citizen without charges or trial.
The Bush administration asserted that in war, the constitution gives the president broad powers. An attorney for detainees counters that the United States has created a "lawless enclave" at the military base in Cuba, where more than 600 men from 44 countries are being held without access to American courts.
The justices sided with the lawyers for the detainees. Their 6 to 3 ruling passes no judgment on the guilt or innocence of the approximately 600 foreign-born men held in the Navy-run prison camp at Guantanamo Bay, Cuba.
Cohen said the ruling may "change the way that the U.S. handles this class of combatants."
The justices also did not address the broad issues of human rights and civil liberties surrounding the prisoners' seizure and detention without trial or guaranteed access to a lawyer.
For now, the high court said only that the men can take the first legal step in contesting U.S. authority to hold them. The men can now presumably take their complaints to a U.S. federal judge, even though they are physically held beyond U.S. borders.
CBS News Foreign Affairs Analyst Pamela Falk said the decision covers not only Guantanamo's use as a military detention facility, but also its role as a migration holding area.
The court on Monday also: