We only know for sure she forced the Ice Nation into the Coalition, otherwise there is a long list of ineffective Commanders. Titus alone served at least four of them before Lexa came into power. Maybe I'm wrong on this, but then at least I'd like to see some comments from the writers that there was no joined clans before at all.
Because the timeline would be weird if there wasn't. Lexa was about 20-22, a bit older than Clarke. She would have been Heda at most six years. If that is an average lifespan for the commanders, that would explain how Titus is now moving on to his sixth. But let's see the details.
She became the Heda at 16. After which there was at least one major war with the Ice Nation.
https://twitter.com/JRothenbergTV/status/604505272703291392
The war would be a trying time. This is probably around the time Costia got murdered as well.
"It was rough, but Lexa held things together. Actually forced the Ice Nation into the coalition after that."
https://twitter.com/JRothenbergTV/status/615011261584019456
So she held the coalition together through the war and forced them into the Coalition.
"Heda is the boss of all 12 Grounder clans. Her own, and 11 others. Lots of blood was spilled to make that so."
https://twitter.com/JRothenbergTV/status/603777378012274689
Heda/Commander is really the Coalition leader.
"Every clan has a leader. Every village within that clan has a leader and all 12 clans bow to the commander."
https://twitter.com/JRothenbergTV/status/610838295614898176
The leaders of the clan listen to the Commander. So it seems to imply previous commanders also had something to lead.
Might be some sort of Roman Empire conquest as well. They conquered, but let them keep part of their beliefs. But this is really a show where I'm wishing for flashbacks.