The most simple way how an employer can violate a work contract is usually not mentioned specifically, but heavily implied: Non-payment of wages.
If the employer does not pay by the due date, he is in breach of the contract, as he had agreed to pay. But what happens then is dependant on the country you are in and the governing laws.
The laws for the protection of the employees are strong in Germany. In the case of nonpayment by due date, the employer automatically is in violation of law and doesn't even need to be notified extra, the moment he doesn't pay he is liable for interest. He's also liable for extra expenses that the employee accrues because the payment is late. If the non-payment continues, the employee may reduce or stop working, depending on the amount of wages outstanding, until the outstanding wages are paid (partially). Neither of these results from the laws are ever referenced in work contracts, but their presence in the law is implied. It also leads to the wording in work contracts such as these:
§1 [What is the employment contract for]
§2 [Start and end of the employment]
Employer will pay employee X as wage per month [sometimes also phrased in an hourly rate].
Payment is due till the last day of a calendar month by money transfer to a bank of employee's choice.
§4 weekly worktime - The regular work time per week is Y hours.
The wording makes it rather clear, that non-payment of the compensation (wage) is a breech of the contract, having the aforementioned effects that are written out in the laws: § 286 Abs.2 Nr.1 BGB & § 614 BGB dictate the interest after the payment date and when that is. § 273 Abs.1 BGB allows the employee that is not paid on time to reduce the work done until the missing payment is done, and it is generally assumed that two full missing monthly wages allow to fully stop working till the missing amount is lower than this. § 626 Abs.1 BGB states, that the employee can, without notice period, dissolve the work contract if 2 monthly wages are not paid, as long as he notified the employer of not having paid on time in the proper way ("Abmahnung" ~ Writeup).
And atop that, as gnasher notes, not paying the employee still forces the employer to pay the income tax on the amount that is owed to the employee, and the tax has even to be paid before the employee is.