QBE is heading for $4.6bn gross written premium (GWP) in the US market through its acquisition of Winterthur US.
The $1.15bn deal with AXA, which was announced last week, is expected to push the insurer into the top 25 of companies in the US.
Combined with the earlier acquisition of Praetorian Financial Group - the US primary specialty subsidiary of Hannover Re in December - it is expected to net QBE profits after tax of more than $300m.
Jim Auden, head of property and casualty at Fitch Rating US, said it is expected QBE will target $4.6bn GWP in the US.
"They are not going to be a St Paul Travelers or an AIG, but the two acquisitions together are pretty significant purchases," Auden said.
But he warned: "The US market has gone through record earnings in 2006, but pricing pressures are maintaining, so there is some uncertainty about future underwriting opportunities."
QBE Group chief executive Frank O'Halloran stated in 2006 that he wanted the group to be the biggest casualty player in the US, and the latest acquisition is another significant push in that direction.
He said: "Winterthur US gives us a strong footprint in western, mid-west and north-east US states for small to medium property and casualty business through a large base of independent agents."
The AXA deal follows the acquisitions of National Farmers Union in 2005 and the One Beacon Agricultural division earlier last year.