The discovery of electricity (and aluminum) has changed the situation with respect to the Iroquois.
My continent has only three sources of aluminum. One is located on land controlled by my ally Ragusa; access to it, however, will have to wait until they get around to converting their farm into a mine. The second source is near Genesse River, on land controlled by the Iroquois, where they already have a mine in place.
The third and final source of aluminum lies on unclaimed territory south of Gao, which is one of my puppet-controlled cities. If I want it quickly, I have a couple of choices. I could annex Gao, taking a hit to my happiness (which is already slightly negative), in order to purchase the tile. Or I could raise another settler and put down yet another city in one of the few regions of open territory anywhere in the world.
You might also note that the Iroquois have ignored my repeated warnings not to settle new cities near me. They have just settled Canandaigua Lake in my sphere of influence. If they plop another city in the gap south of Gao, they could grab the aluminum and keep it from me.
Moreover, I can see two more Iroquois settlers heading down the east coast looking for more land to steal.
As noted earlier, my troops are already in place along the Iroquois border. And everything has been upgraded to artillery and foreign legions.
The time has come to crush the Iroquois.