If a foreign company carrying on an insurance business within the United States would qualify under part I or II of this subchapter for the taxable year if (without regard to income not effectively connected with the conduct of any trade or business within the United States) it were a domestic corporation, such company shall be taxable under such part on its income effectively connected with its conduct of any trade or business within the United States. With respect to the remainder of its income which is from sources within the United States, such a foreign company shall be taxable as provided in section
881.