Thanos has built an army out of Earth's heroes and sent them to invade and burn another world. However, Reed managed to fake his own death, and comes back with the rest of the FF to finish things. With Thanos being the only one in direct contact with the cosmic cube, things appear hopeless. Their hopelessness, however, depends on the idea that Reed didn't think about or prepare for an eventuality such as this.
The phrase "Reed didn't think" is rarely an accurate description in any situation.
