First Contact
The UNSC Steppe Runner, an Autumn class Heavy Cruiser was on a shakedown cruise when they encountered the UES (United Earth Starship) George Hammond under Colonel Samantha Carter. It happened on the 6th of June 2015. Captain Jacob Slynt of the Steppe Runner was deeply suspicious of the George Hammond. After a standoff President Redfield-then on her first term-sent a diplomatic team to negotiate and after a meeting with Colonel Carter they met with a diplomatic team from Earth a week later. The negotiation took place on board the UNSC Infinity as a symbolic show of strength.
The negotiations stretched out due to division in the UEG, that had recently been granted the powers seized decades before. The Assembly-divided between civilians and military officers, and outer colonial governors-argued fiercely. Hardliners argued the annexation of New Earth, moderates argued peaceful relations and pacifists argued to work for friendship and an alliance with New Earth; hoping for military aid in the Covenant Civil War. In the end President Francesca Redfield had diplomats sign a treaty to preserve a form of status quo and established embassies on each other's homeworlds. Stargate Command introduced them to the Free Jaffa Nation.
In September of the same year a Sword of Sanghelios ship explored to discover new resources for the war and came across an impoverished Free Jaffa planet. Unbeknownst of the planets owner they brought a garrison through the Stargate. A Stargate had been found of Sanghelios some years prior but it was never fully understood and some thought it heresy since it wasn't Forerunner and hid it away. The Jaffa High Council's response was hard and swift, though some like Councilor Teal'c argued for a measured and diplomatic response, since they didn't know who they were exactly dealing with. But he knew it was important for the High Council to be united in responding to a threat and he voted for military retaliation to avoid splitting the newly reformed Council on Dakara. On the ground the Sangheili proved the better warriors but the Jaffa's numbers overwhelmed them and the orbiting Frigate that discovered the planet was destroyed by a fleet of Ha'taks.
Arbiter Thel Vadam's subordinates argued for a military response but since he knew little about the situation he refused to escalate further. His conclusion was that he couldn't risk war with anyone until the Civil War ended. He reached out to his UNSC allies, who in turn reached out to their New Earth allies and there was a conference between the UNSC, New Earth, the Free Jaffa and the Sword of Sanghelios. After long and hard negotiations they founded an alliance between them. Some months later the Tok'ra joined.
The Jaffa could recognize themselves slightly in the Sangheilis plight. Primarch Amenset sent a token expeditionary force to fight alongside Arbiter Vadam's forces in early January of 2017. This expedition was gradually expanded with ships and ground forces.