Attack represents another security failure for the group reeling from repeated Israeli blow
Attack comes as militant group says two days of pager and walkie-talkie explosions amount to declaration of war
Number of casualties appears to top 1,000, according to a Hezbollah official
The fighting with Israel has destroyed part of the country and spared the other, but the divide might not hold
U.S. special envoy Amos Hochstein was in the Middle East seeking to head off a broader regional conflict