These guys packed quite a lunch bag for their Independence Day barbecue: spaceships the size of Hawaiian islands, shields that could withstand the nuclear bombing of Houston, weapons capable of leveling Washington D.C. in a single blast, plus all the information about human anatomy they could possibly probe out of Randy Quaid's a*****e.
But, after pancaking dozens of cities, a rag-tag group of humans cause each and every one of the alien ships to crash, the thousands of people who were no doubt crushed under each one a small price to pay for victory.
But We Forgot About...
Let's be generous and assume that every one of the alien city-destroying mother ships was downed. Do you have any idea how large a 15-mile-wide spaceship is? Each one is like a flying city, 1,000-stories high and about 100 blocks wide in every direction. And while New York may not have been designed to have giant spaceships fall on it, we have to imagine the space crafts had been designed with a contingency plan for gravity.