BEIRUT—Saudi Arabia pledged $3 billion to bolster Lebanon's armed forces, in a challenge to the Iranian-allied Hezbollah militia's decadeslong status as Lebanon's main power broker and security force.

Lebanese President Michel Sleiman revealed the Saudi gift on Lebanese national television Sunday, calling it the largest aid package ever to the country's defense bodies. The Saudi pledge compares with Lebanon's 2012 defense budget, which the Stockholm International Peace Research Institute put at $1.7 billion.

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