The Command to be Patient and Good News of Victory
Here Allah commands His Messenger to patiently bear the rejection of those who rejected him: `Allah will fulfill His promise to you that you will be victorious and will prevail over your people, and you and those who follow you, will be the successful ones in this world and the Hereafter.'
فَـإِمَّا نُرِيَنَّكَ بَعْضَ الَّذِى نَعِدُهُمْ
(and whether We show you some part of what We have promised them,) means, in this world, and this is what happened, for Allah gave them the joy of humiliating the leaders and nobles (of the Quraysh), who were killed on the day of Badr, then Allah granted them victory over Makkah and the entire Arabian Peninsula during the lifetime of the Prophet .
أَوْ نَتَوَفَّيَنَّكَ فَإِلَيْنَا يُرْجَعُونَ
(or We cause you to die, then still it is to Us they all shall be returned.) means, `and We shall inflict a severe punishment upon them in the Hereafter.' Then Allah says, consoling His Prophet :
وَلَقَدْ أَرْسَلْنَا رُسُلاً مِّن قَبْلِكَ مِنْهُم مَّن قَصَصْنَا عَلَيْكَ
(And, indeed We have sent Messengers before you, of some of them We have related to you their story.) as Allah also says in Surat An-Nisa', meaning, `We have revealed the stories of some of them and how their people disbelieved in them, but the Messengers ultimately prevailed.'
وَمِنْهُمْ مَّن لَّمْ نَقْصُصْ عَلَيْكَ
(And of some We have not related to you their story, ) and they are many, many more than those whose stories have been told, as has been stated in Surat An-Nisa'. Praise and blessings be to Allah.
وَمَا كَانَ لِرَسُولٍ أَن يَأْتِىَ بِـَايَةٍ إِلاَّ بِإِذْنِ اللَّهِ
(and it was not given to any Messenger that he should bring a sign except by the leave of Allah.) means, none of the Prophets was able to bring miracles to his people except when Allah granted him permission to do that as a sign of the truth of the message he brought to them.
فَإِذَا جَـآءَ أَمْرُ اللَّهِ
(But, when comes the commandment of Allah,) means, His punishment and vengeance which will encompass the disbelievers,
قُضِىَ بِالْحَقِّ
(the matter will be decided with truth,) so the believers will be saved and the disbelievers will be destroyed. Allah says:
وَخَسِرَ هُنَالِكَ الْمُبْطِلُونَ
(and the followers of falsehood will then be lost.)
Tafsir Ibn Kathir (abridged, English).