Promises are of course easily given and equally easily broken. So easily ignored it became necessary through time and sad in adherence to increase the level of a simple promise to something more intransient, a pledge, an oath, a plight.
Of course the importance of personal honesty is always paramount, neither promises, or any other kind of assurance can override simple deviousness, even when the giver of the guarantee endorses their covenant by accepting some terrible repercussion for failing in their bond.
A pledge tends to be given to an inanimate object, a nation, a flag, something that has an idealistic meaning, inherent moral or ethical stances, which unfortunately often fails to rise to the committers expectations.
Oaths are contradictory, in one instance being the most committed of promises, sworn under the auspices of some relic of religious significance. Equally the word can indicate some expletive of deep and dark origin, both failing to bring any guarantee of success.