And that He might make evident those who are hypocrites. For it was said to them, “Come, fight in the way of Allah or [at least] defend.” They said, “If we had known [there would be] fighting, we would have followed you.” They were nearer to disbelief that day than to faith, saying with their mouths what was not in their hearts. And Allah is most Knowing of what they conceal – Quran 3:167
128 Only a fight in self-defence – in the widest meaning of this term – can be considered a ‘fight in God’s cause’ (see 2: 190-194, and the corresponding notes); and thus, the practice ‘or’ between these two phrases is almost synonymous with the expression ‘in their words’.
129 This is an allusion to the three hundred men who, on the way from Medina to Mount Uhud, forsook the Prophet on the specious plea that he did not really intend to give battle (see note 90 above). But since they knew in their hearts that it would come to a fight, their defection from God’s cause almost amounted to a denial of Him (kufr, here rendered as ‘apostasy’). 
And that He might also know the hypocrites, and those who, when it was said to them, after they had fled the fighting, namely, ‘Abd Allāh b. Ubayy and his companions: ‘Come now, fight in the way of God, His enemies, or defend’, us against the enemy by increasing the multitude [of our fighters], if you are not going to fight; they said, ‘If we knew how, [if] we were skilled enough, to fight we would follow you’. God then said, showing them to be liars: They that day were nearer to unbelief than to belief, for what they manifested of their forsaking the believers, whereas before they had been outwardly nearer to belief; saying with their mouths that which was not in their hearts, for even if they had known how to fight they would not have followed you. And God knows best what they hide, of hypocrisy. 
Malik Ghulam Farid
523. Trials and tribulations are intended to distinguish true believers from those weak of faith. In this way, the sufferings of the Muslims at Uhud proved a blessing in disguise. They served to distinguish the true believers from the hypocrites who had so far remained mixed up with true believers.
524. The particle au rendered as ‘and’ literally means ‘or’ and is equivalent to ‘in other words,’ or ‘what is the same thing as’ etc.
525. The expression, lau Na’lamu Qitalan,, may mean:
(1) If he knew that there would be fighting i.e., we knew that there would be no fighting and that the Muslims would at once run away before their very powerful enemy without giving fight.
(2)If we knew it to be a fight, i.e., it was no fight in which the Muslims were going to be engaged but rather their certain destruction in view of the appalling difference between the numbers and equipment of the opposing forces.
(3) If we knew how to fight. In this case, the words may be taken to have been spoken ironically, signifying: ‘We are unaware of the art of war; if we had been acquainted with it, we would have fought along with you.’
The allusion in the verse is obviously to the defection at Uhud of a party of 300 Hypocrites under their leader ‘Abd Allah b.Ubbay, who deserted the Muslims and went back to Medina. 
Maulana Muhammad Ali
167a. The words show clearly that in the language of the Qur’an, fighting in Allah’s way carries the significance of fighting in self-defence.
167b. They feigned they did not know how to fight. Or the meaning may be, if we knew that it was a fight, the implication being that the Muslims were going, not to fight but to sure destruction with such a disparity of numbers. 
 The Message of The Quran translated and explained by Muhammad Asad page 147
 Tafsir al-Jalalayn – http://www.altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=74&tSoraNo=3&tAyahNo=167&tDisplay=yes&UserProfile=0&LanguageId=2
 The Holy Qur’an Arabic Text With English Translation & Short Commentary by Malik Ghulam Farid page 170
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali page 182