By Paul Nassar, NBC News

BEIRUT, Lebanon – It was the world’s worst kept secret. Not anymore.

For two years, world leaders and intelligence services knew that Hezbollah, the militant Lebanese Shiite group, was involved in the ever-expanding civil war in neighboring Syria.

Long-time allies of Bashar Assad, Hezbollah and its leadership threw their weight behind the embattled Syrian president.

They never admitted, however, that they were actively participating in the battles that continue to tear Syria apart. But in a speech Saturday to commemorate the 13th anniversary of the Israeli withdrawal from south Lebanon, Hezbollah’s Secretary General formally announced the group’s involvement in the Syrian war.

Syria saw one of the deadliest days of fighting in its civil war Saturday. Meanwhile, the leader of the powerful Lebanese militant group Hezbollah said his fighters would wage an all-out battle to save President Bashar Assad. NBC’s Richard Engel reports.

“We will continue to the end of the road. We accept this responsibility and will accept all sacrifices and expected consequences of this position,” Hezbollah’s leader Sayyed Hassan Nasrallah said in a televised speech on Saturday evening. “We will be the ones who bring victory.”

Nasrallah painted the Syrian rebels not as freedom fighters trying to overthrow a brutal dictatorship, but as stooges of the United States and Israel who were trying to weaken Syria to advance the Jewish state’s interests. The war, he said, was a plot devised by the U.S. and its allies to weaken the resistance against Israel.

“Syria is no longer a place where there is a popular revolution against a political regime,” he said. “Rather it has become a place for imposing a political plan led by America and the West, and its tools in the region.”

Nasrallah maintained that Hezbollah was fighting to stop radical Sunnis, know as Takfiris, from overthrowing an ally who has stood shoulder-to-shoulder with them in their battle against Israel. By reverting to this time-tested rhetoric of the Arab world vs. Israel, Nasrallah was clearly trying to deflect attention from the sectarian nature of Hezbollah’s involvement.

But in a possible sign that Nassrallah’s statements had indeed inflamed sectarian tensions, two rockets hit a Hezbollah stronghold in southern Beirut on Sunday. It was the first attack since the outbreak of the two-year Syrian conflict.

Nasrallah claims that his followers had a duty to fight a jihadi battle against those trying to bring down the Syrian regime. Paramount in this fight is the battle currently taking place in Qusair  — a strategic Syrian town that hugs the Lebanese border. Syrian rebels are hunkered down, surrounded on all sides by Assad’s forces and Hezbollah fighters.

Why is Qusair pivotal? Its proximity to the Lebanese border means that it is a major conduit of arms from Lebanon to the rebels. The town also straddles one of the main highways linking Damascus to the Alawite coastal strongholds in northern Syria. By capturing Qusair, Assad will effectively cut in two the regions held by the rebels, thus dealing them a severe blow.

The fear many have is that the Syrian crisis spills over into Lebanon — a fragile country whose own sectarian mix is as explosive as Syria’s. Indeed, many claim that the spill-over has been happening for months, with battles raging in the northern Lebanese city of Tripoli. So far though, these battles have been somewhat contained.