If the net inflow of foreign investment were to diminish, the United States would no longer be able to invest so much more in the domestic capital stock than it saves.
The net inflow of foreign investments has been on the rise. 
mnli hypothesis: The net inflow of foreign investments has been on the rise.  premise: If the net inflow of foreign investment were to diminish, the United States would no longer be able to invest so much more in the domestic capital stock than it saves.