  We discuss the events that led to the giant eruption of Eta Carinae, and find
that the mid-nineteenth century (in 1838-1843) giant mass-loss outburst has the
characteristics of being produced by the merger event of a massive close
binary, triggered by the gravitational interaction with a massive third
companion star, which is the current binary companion in the Eta Carinae
system. We come to this conclusion by a combination of theoretical arguments
supported by computer simulations using the Astrophysical Multipurpose Software
Environment. According to this model the $\sim 90$\,\MSun\, present primary
star of the highly eccentric Eta Carinae binary system is the product of this
merger, and its $\sim 30$\,\MSun\, companion originally was the third star in
the system. In our model the Homunculus nebula was produced by an extremely
enhanced stellar wind, energized by tidal energy dissipation prior to the
merger, which enormously boosted the radiation-driven wind mass-loss. The
current orbital plane is then aligned with the equatorial plane of the
Homunculus, and the symmetric lobes are roughly aligned with the argument of
periastron of the current Eta Carina binary. The merger itself then occurred in
1838, which resulted in a massive asymmetric outflow in the equatorial plane of
the Homunculus. The 1843 outburst can in our model be attributed to the
subsequent encounter when the companion star (once the outer most star in the
triple system) plunges through the bloated envelope of the merger product, once
when it passed periastron again. We predict that the system has an excess space
velocity of order 50\,km/s in the equatorial plane of the Homunculus. Our
triple model gives a viable explanation for the high runaway velocities
typically observed in LBVs \citep{2015MNRAS.447..598S}.
