This is quite an interesting question. The answer will be affected by several things you would have to consider.
The first of which would be the identity of your rising dictator. Who is he? Is he a member of the founding families, of your 'nobility'? Or is he a commoner, someone rising from the 'people'?
A noble would have the advantage of wealth and ties, of social powers. However, since you implied that the 'nobles' are blamed for the war, it would be a drawback to overcome.
A commoner would not have the social power and the ear of the nobility. He would be an outsider and someone the nobles would seek to eliminate or control. However, he would have the advantage of the people's solidarity, a hero rising from the ranks.
In any case, this man would have to be cunning and ruthless when needed, but in a covert way. He would have to win the support of the people AND the founder families. It would mean he would have to be politically savvy.
The first thing he would have to do is to understand his opponents and goals:
1) What are the weaknesses and wants of the noble families?
Weaknesses can be exploited. He would have to threaten some, cow others, bribe, deceive or win the true support of these families. He can even pull the Killer Puppet stunt: pretend he can be ruled, that he's stupid or easily controlled and would be a good puppet ruler, then turn on his puppeteers when they are no longer needed.
2) What are the needs and desires of the people?
Find out what the people want and give it to them, or even pretend to, and you will have their support. He will have to make the people believe his rule is in their best interest. He can pull a Patriot stunt: establish himself as the liberator of the people from the oppression of the founder families. This may alienate the nobles though, so either he takes that into account or he does it carefully to avoid such a thing.
3) Is there an outside faction or enemy that can be exploited?
If he can unite the people against a common enemy or use that enemy as a target he would eliminate or act against, he would become a hero and the obvious choice for a ruler. He can even pull a Phantom Killer stunt: fabricate an enemy or fabricate events to turn an uninvolved outside faction or nation into an enemy in the eyes of the people and nobles.
4) Is there any resource or sought-after item or knowledge he can bring the people?
Every nation covets something. Whether its conquest, more land, some innovation or invention they do not yet possess, something thought out of their reach or something that has not yet been obtained before. If he can bring them something like that, he would have the nation's ear.
5) Is there an option of military takeover?
When all else fails, there is always the good old hostile takeover. If he has the means, the wealth or the right to recruit or otherwise beget a big or strong enough army, he can just become a true tyrant and take everything by force. A crude solution but sometimes the brute force approach is the easiest. How he begets that army can actually be a matter for another such detailed debate.