Concise writing is the key to clear communication. Wordiness obscures your ideas and frustrates your reader. Make your points as succinctly as possible, and move on. As Strunk and White tell us in Elements of Style:
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences.... This requires not that the writer make all sentences short, or avoid all detail and treat subjects only in outline, but that every word tell. (23)
Once you start searching for unnecessary words, you will find you can cut many without any loss of meaning. In fact, your writing will be crisper and more appealing. Remember: make "every word tell."
The reason that General Lee invaded Pennsylvania in June, 1863, was to draw the Army of the Potomac away from Richmond.
revised (replace was with action verb invaded)
General Lee invaded Pennsylvania in June, 1863, to draw the Army of the Potomac away from Richmond.
Tip: As a first step in reducing wordiness, identify instances of this is, there are, and it is at the beginning of your sentences, and ask yourself whether you can eliminate them.
In Crew's argument there are many indications of her misunderstanding of natural selection.
revised (replace subject there with argument; replace verb are with demonstrates.)
Crew's argument repeatedly demonstrates misunderstanding of natural selection.
In most instances, it is better to put verbs in the active voice. Passive voice produces unclear, wordy sentences, whereas active voice produces clearer, more concise sentences.
In 1935 Ethiopia was invaded by Italy.
In 1935 Italy invaded Ethiopia. (more concise and vigorous)