Bhagavad Gita

ಕರ್ಮಣ್ಯೇವಾಧಿಕಾರಸ್ತೇ ಮಾ ಫಲೇಷು ಕದಾಚನ ।
ಮಾ ಕರ್ಮಫಲಹೇತುರ್ಭೂರ್ಮಾ ತೇ ಸಂಗೋSಸ್ತ್ವಕರ್ಮಣಿ ॥೪೭॥
karmaṇyevādhikāraste mā phaleṣu kadācana |
mā karmaphalaheturbhūrmā te saṃgoSstvakarmaṇi ||47||

Gist of the sloka:
You have right to the performance of action and not to the rewards. Don’t do the karma [action] keeping the rewards as the sole objective.
This is one of the most famous verse, grossly mis-interpreted over eons. It is confusing as it says one should do the action compulsorily but not in anticipation of rewards. The very motivation for karma is now under question. Unfortunately, these cryptic verses has never been fully understood by the populace.
The simple fact is Lord Krishna is telling, while its necessary for us to do our duty, we should not stake a claim to insisting on a particular reward. The right to duty remains with us while the Lord retains the right to reward. We should not over reach ourselves in acquiring the Lord right to ourselves. Lord knows at all times what to be given as reward and when. We should always be within our limits and boundaries.
The question would then be, is it wrong to aspire for a reward. No, it’s not. Only difference being we request the Lord for a reward not demand it as a right. We are not omnipotent, omnipresent or omniscient as Lord. Only he has to the right to reward. It is like a child writing the exam and determining for itself the reward of marks. We don’t allow it ourselves to our children and we separate the action [writing exams] from rewards [marks] and in higher plane it is exactly the same.
To insist on the exact reward and putting a condition before doing our duty, is a strictly not allowed. It would border on arrogance in front of the universal lord. We in our ignorance sometime insist on a particular reward from the Lord. The Lord knowing what is good for our progress in self-realization [for that is the ultimate goal for all lives] decides and then offers us His reward. Not realizing this, we go into grief [mental dissatisfaction, disturbance] as rewards is not according to our dreams. If we accept what is provided, then there is no grief.
One should not withdraw from performing assigned duties at any time. For then there is no grief. One who sees win/defeat in equal manner, have no reason to grieve. This state will elevate us to higher level of consciousness. We therefore learn more in loss than when we are winning. The Lord gives us rewards with the sole aim to uplift us and sometimes rewards might appear not give us any happiness. With acceptance, this aspect will never bother anyone.
Performing ones given actions, without seeking specific rewards but with the sole aim to please the Lord, is Lord Krishna karma yoga.
Arjuna fighting the war with the aim to win for righteousness is not wrong. What is wrong is to insist that he should win or to worry about loss. He should therefore surrender to the will of the Lord, the results/actions of the war; and only perform his duty faithfully. In this state of mind, the ups and downs of the various battles in the war would not affect him. Accepting whatever the Lord provides as his blessings is the safest and surest way to reach the Lord.