Allaah سبحانه و تعالى said, "There is no compulsion in religion. Verily, the right path has become distinct from the wrong path."'
Imaam Ibn Katheer رحمه الله wrote,
"Meaning, do not force anyone to become Muslim, for Islaam is plain and clear, and its proofs and evidence are plain and clear. Therefore, there is no need to force anyone to embrace Islaam. Rather, whoever Allaah سبحانه و تعالى directs to Islaam, opens his heart for it and enlightens his mind, will embrace Islaam with certainty. Whoever Allaah سبحانه و تعالى blinds his heart and seals his hearing and sight, then he will not benefit from being forced to embrace Islaam.
It was reported that the Ansaar رضي الله عنهم were the reason behind revealing this aayah, although its indication is general in meaning. Ibn Jareer رحمه الله recorded that Ibn 'Abbaas رضي الله عنهما said [that before Islaam], 'When (an Ansaari) woman would not bear children who would live, she would vow that if she gives birth to a child who remains alive, she would raise him as a Jew. When Banu An-Nadeer (the Jewish tribe) were evacuated from Al-Madeenah, some of the children of the Ansaar رضي الله عنهم were being raised among them, and the Ansaar رضي الله عنهم said, "We will not abandon our children." Allaah سبحانه و تعالى revealed, "There is no compulsion in religion. Verily, the right path has become distinct from the wrong path."'
(Sooratul-Baqarah, 2:256) 
Abu Dawood رحمه الله and an-Nasaa'ee رحمه الله also recorded this hadeeth. 
As for the hadeeth that Imaam Ahmad رحمه الله recorded, in which Anas رضي الله عنه said that the Messenger of Allaah صلى الله عليه وسلم said to a man,
«أَسْلِم» قَالَ: إِنِّي أَجِدُنِي كَارِهًا قَالَ: «وَإِنْ كُنْتَ كَارِهًا» 'Embrace Islaam.'
The man said, 'I dislike it.' The Prophet صلى الله عليه وسلم said, 'Even if you dislike it.'
First, this is an authentic hadeeth, with only three narrators between Imaam Ahmad رحمه الله and the Prophet صلى الله عليه وسلم. However, it is not relevant to the subject under discussion, for the Prophet did not force that man to become Muslim. The Prophet صلى الله عليه وسلم merely invited this man to become Muslim, and he replied that he does not find himself eager to become Muslim. The Prophet صلى الله عليه وسلم said to the man that even though he dislikes embracing Islaam, he should still embrace it,`for Allaah سبحانه و تعالى will grant you sincerity and true intent.'"  Notes:
 At-Tabari 5:407.
 Abu Dawood 3:132, An-Nasaa'ee in al-Kubraa 6:304.
 Ahmad 3:181.
 Tafsir Ibn Kathir (Abridged), Darussalam, vol. 2, pp. 30-31.