The ex-prime minister made clear that he disagreed with the Labour leader's handling of the issue, warning that the country could become a breeding ground for extremists if Bashar Assad's regime was not confronted.
However, he also admitted that the long and bloody struggle which followed the invasion of Iraq had left Britain "hesitant" about getting involved.
In an interview with the BBC, Blair said he was "disappointed" that the Commons had rejected a Government motion paving the way for direct action.
"This is something where I just have to disagree with the leadership of the party," he said. "I know it's a difficult position for political leaders to be put in when they have got to take decisions like this."
Blair said without foreign intervention "you will have an Assad-dominated state, and that means in this instance an Iran-dominated state, probably around the borders of Lebanon and controlling most of the wealth of Syria".
"Then you'll have a larger geographical hinterland to the east that will be controlled by various Sunni groups, most of whom are likely in these circumstances to be extreme, and you could have a breeding ground for extremism actually much worse and much more potent than Afghanistan," he added.
Blair said there was no question that chemical weapons had been used, and that Assad's forces were responsible.
"The truth is, the reason why Iraq makes us hesitant is because Iraq showed that when you intervene in the circumstances, where you have this radical Islamist issue, both on the Shia side and the Sunni side, you are going to face a very difficult, tough conflict."
Meanwhile, former culture secretary Ben Bradshaw said many Labour MPs felt "uneasy" that the UK's options had been limited after the Government's shock defeat in the Commons.
Writing in the Guardian, he made clear that he blamed David Cameron for "petulantly" ruling out direct intervention immediately after losing the vote.
But he also expressed alarm that Miliband was following the Prime Minister's lead rather than taking a "sensible and measured approach".