Sheikh Mujibur Rahman, also known as the "Father of the Nation" in Bangladesh, played a pivotal role in declaring East Pakistan as an independent republic named Bangladesh. On March 26, 1971, he issued a declaration of independence from East Pakistan, which was under Pakistani rule at the time. This declaration marked the beginning of the Bangladesh Liberation War, leading to the eventual establishment of Bangladesh as an independent nation. Answer: $\boxed{\text{Sheikh Mujibur Rahman}}$