While you shouldn't need to at lower difficulties or on easier campaigns, you can always quick save before a move or at the end of your turn and reload if you didn't like the result of your move or the enemy moves. Do it a lot in the early scenarios to build up hero unit reserves and see if you can make it through later scenarios without reloading. Mark the turn start save and try a strategy, if it fails go back to that turn instead of starting over. And note that if your strategy is poor, continually reloading alone won't let you win. (Unless maybe you spend a crap ton of time waiting for positive outcomes of minuscule-probability events...)
Edit: I remembered a quote I liked from early on in Iain Banks' The Player of Games. While I still like chess (and take issue with the notion that reality is built on chance, rather than our predictive measurements of reality restricted to chance due to us being part of reality not outside it) I thought this was a pretty great put-down of games without chance at all:
"All reality is a game. Physics at its most fundamental, the very fabric of our universe, results directly from the interaction of certain fairly simple rules, and chance; the same description may be applied to the best, most elegant and both intellectually and aesthetically satisfying games. By being unknowable, by resulting from events which, at the sub-atomic level, cannot be fully predicted, the future remains make-able, and retains the possibility of change, the hope of coming to prevail; victory, to use an unfashionable word. In this, the future is a game; time is one of the rules. Generally, all the best mechanistic games - those which can be played in any sense "perfectly", such as a grid, Prallian scope, 'nkraytle, chess, Farnic dimensions - can be traced to civilisations lacking a realistic view of the universe (let alone the reality). They are also, I might add, invariably pre-machine-sentience societies.
"The very first-rank games acknowledge the element of chance, even if they rightly restrict raw luck. To attempt to construct a game on any other lines, no matter how complicated and subtle the rules are, and regardless of the scale and differentiation of the playing volume and the variety of the powers and attibutes of the pieces, is inevitably to schackle oneself to a conspectus which is not merely socially but techno-philosophically lagging several ages behind our own. As a historical exercise it might have some value. As a work of the intellect, it's just a waste of time. If you want to make something old-fashioned, why not build a wooden sailing boat, or a steam engine? They're just as complicated and demanding as a mechanistic game, and you'll keep fit at the same time."