Bulgarian investigators recently issued their report on the terrorist bombing of Israeli tourists in Bulgaria last year. The Bulgarians agreed with an earlier Israeli investigation that the attack was carried out by the Iran-backed Lebanese Hezbollah terrorist organization. Hezbollah is a Shia radical group that is obsessed with destroying Israel and turning Lebanon into a Shia religious dictatorship. The two main suspects involved appear to be hiding out in Lebanon, where Hezbollah controls the southern part of the country. Authentic Australian and Canadian passports were used by the attack planners to get in and out of Bulgaria. This brought to light decades of Hezbollah efforts to establish members overseas, especially in Canada and Australia, where legitimate passports could be obtained and then used to help carry out international terror attacks. Canada has discovered that there are thousands of their passports held by people who spend most of their time outside of Canada. Israel and Bulgaria also want the EU (European Union) to declare Hezbollah a terrorist organization. The U.S. and Israel have already done this, but local politics and anti-Israeli attitudes have prevented such a move in Europe.
Meanwhile Hezbollah is facing hard times because of the rebellion next door in Syria. There, an Iran backed secular dictatorship has been under attack by rebellious Syrians for two years and the government is losing. A new government would be hostile to Iran and Hezbollah. Syria has been a supply line (from Iran) for Hezbollah as well as a refuge from Israeli attack. That will all be gone, and Hezbollah's many enemies (mostly in Lebanon but most specifically Israel) will take advantage of this and try to destroy a weakened Hezbollah.
Hezbollah, with better access to the world (than heavily sanctioned Iran), has long been carrying out terror attacks (mostly against Israel) for Iran. As a result of that, Hezbollah has, over the last two decades, become identified and treated as an international terrorist organization by most intelligence agencies in the West.
In the last few years Iran has been desperate to strike back at Israel. That's because Israel has carried out numerous attacks against the Iranian nuclear weapons program. This includes the assassination of Iranian scientists working on that program plus Cyber War attacks on Iranian facilities. There have been other operations outside Iran to foil Iranian smuggling (of military and nuclear weapons components) operations. Iran wants revenge and has not been getting much. The Bulgarian attack may have more blowback than Iran anticipated.