"When the terrorists are capable of doing such acts," he said, "they must know that they will face a France very determined."
After January's attack, Prime Minister Manuel Valls declared France at "war against terrorism, against jihadism, against radical Islamism."
But the scale of Friday's assaults takes that war to a new level, according to experts.
Valls tweeted: "We are at war. We will take exceptional measures. And this war, we will win."
Hollande said the "act of war" was carried out by "a jihadist army, against France, against the values that we're defending everywhere in the world."
"It's an act of war prepared, organized and planned from outside with inside accomplices," Hollande said. "It's an act of absolute barbarity."
On Friday, he vowed a tough response. "We will lead the fight and we will be ruthless, and we had to be here among the people who were subject to these atrocities because when the terrorists are capable of doing such acts they must know that they will face a France very determined -- a France united."
Will response be a military one?
Increasingly, France has moved in the direction of military force against terrorists overseas in recent years, according to Christopher Chivvis, associate director of the International Security and Defense Policy Center at the RAND Corp.
That's a significant change from a decade ago when France focused primarily on the domestic front in counterterrorism strategy.
"France can be one of the most aggressive countries in the world when it comes to striking terrorist groups overseas." Chivvis said via email Saturday.
"President Hollande has considerable capability at his disposal, including advanced airpower, highly trained special forces, and land and naval assets. France needs support of NATO allies and especially the United States to employ these capabilities to their fullest, however."
It's clear France will respond militarily, according to CNN military analyst Lt. Col. Rick Francona.
"The problem will be, how are they going to do that?" Francona said. "Stepping up air attacks doesn't do anything unless we got adequate targets to strike. That's been the problem all along in Syria and Iraq."
Christiane Amanpour, CNN's chief international correspondent, described Hollande's remarks Saturday as "a language of war."
But David Schanzer, director of the Triangle Center on Terrorism and Homeland Security and a professor of public policy at Duke University, said unilateral action by France against ISIS in Iraq or Syria is unlikely.
Instead, Schanzer said via email, France may increase its activity and visibility as part of the international coalition against ISIS.
Yes, the massacre will increase calls for more vigorous military action than the coalition has mustered to date, Schanzer said.
"But despite the horrific attacks, the reasons against a large-scale land invasion by U.S. and NATO forces against ISIS in Iraq and Syria remain," he said.
"Such an invasion will deepen the extremist narrative of clash of civilization between the West and Muslims, will insert our militaries into a deep, nasty and unwinnable civil war, and the invading force will eventually be responsible for reconstructing a semblance of order and governance in (a) chaotic region infected with sectarian divisions."
What steps did France take after Charlie Hebdo?
Cherif Kouachi, 32, and Said Kouachi
, 34, the brothers who authorities said carried out the Charlie Hebdo shootings, were French citizens known to the country's security services
, according to officials.
One spent time in jail for ties to terrorism, and was in Syria as recently as last year,
according to a French source. The other went to Yemen for training, officials say. They were killed in January during a standoff northeast of Paris.
"One of the Kouachi brothers had trained with al Qaeda in Yemen, but that had been some time in the past and it was a largely homegrown attack -- although with some links to al Qaeda in Yemen," CNN's national security analyst Peter Bergen said.
"Of course, France didn't attack, as far as I can tell, al Qaeda in Yemen. But the scale of this is obviously quite different. And the fact is, there is an ongoing war in Syria to which the French are attached. So you can see that they could amp up their role if they felt so inclined."
How has France fought ISIS before?
In an online statement distributed by supporters Saturday, ISIS said eight militants wearing explosive belts and armed with machine guns attacked precisely selected areas in the French capital.
A Syrian passport was found near the body of an attacker outside one of the targeted sites, the Stade de France, according to a police source, CNN affiliate France 2 and other French media reported.
"George W. Bush called 9/11 an act of war and as a result conducted an actual war or two," Bergen said.
"So given France's quite aggressive campaign in Mali against al Qaeda in the Islamic Maghreb and given the fact that France was one of the leaders of the coalition to overthrow (the late Moammar) Gadhafi in Libya, it seems quite likely that France will respond aggressively."
It has responded with military might in the past.
In September, the French military carried out its first airstrikes against ISIS in Syria,
according to the French President's office.
France announced that month that it was expanding its aerial campaign against ISIS in Iraq -- which it began a year ago -- to include the militant group's positions in Syria.
The President's office said the strikes in Syria were based on intelligence gathered from air surveillance operations conducted over Syria.
What are other examples of French military force?
France has also conducted military strikes in support of government forces battling militant Islamist forces in Mali,
a French colony until 1960.
Islamist militant Abdelhamid Abou Zeid was killed in 2013
in fighting by French forces in northern Mali. Abou Zeid was one of the major figures in al Qaeda in the Islamic Maghreb, al Qaeda's North African offshoot.
In December, French troops killed a senior jihadist leader in northern Mali -- a man purportedly behind a number of high-profile attacks and kidnappings, according to the French military.
The United States had issued a $5 million reward for Ahmed el Tilemsi, the military head and co-founder of a jihadist group called the Movement for Unity and Jihad in West Africa.
In 2012, France pulled its last troops engaged directly in combat out of Afghanistan
. At the time, about 1,500 French troops remained in Afghanistan to remove equipment and to help train Afghan forces. France was one of the bigger contributors of troops to the NATO-led International Security Assistance Force.
"In its war on (al Qaeda) in Mali, French forces struck with great speed, force and violence against the enemy," said Chivvis of the RAND Corp.
"They could do so again, but to have the same effect against Daesh (ISIS), they would need a much bigger operation, hence much more support from other countries. It remains to be seen if that support will be forthcoming."
Will France's response be different this time?
That depends on where the investigation leads French authorities.
"At the end of the day it seems unlikely that this is simply a wholly homegrown incident given the scale and complexity of the attack," Bergen said. "It would seem likely that it would have some links to ISIS central and therefore that would lead you to take aggressive action."
Schanzer, the Duke professor, said the basic contours of the current strategy against ISIS are right.
That means to keep encouraging and supporting local forces to fight, conduct airstrikes against ISIS targets and maintain pressure against both ISIS and Syrian President Bashar al-Assad to force a political settlement in Syria, according to Schanzer.
"ISIS will remain a formidable force until a political settlement takes the steam out of the sectarian conflict in Syria," he said.
ISIS wants to satisfy its blood lust fighting against Americans on its home turf, Schanzer said.
"However, when ISIS is committing atrocities against or being attacked by other Muslims, it has a much harder time explaining how it is advancing the cause of Muslims or representing Islam in any comprehensible way."