'Stratagem' 'A stratagem'

Neither; both are comma-splice sentences, and stratagem is countable. Use:

You tried to trick me; it was a stratagem.

Thank you very much, Mica.
When do we exactly use comma-splice?

A comma-splice is:

The mistaken effort to link two complete sentences with a form of punctuation inadequate to the task. "He had no evidence to support his argument, therefore he lost the debate to his rival" is a comma splice. The comma should be replaced by a semicolon or a period.

WesternAmericanThank you very much, Mica.
When do we exactly use comma-splice?

You tried to trick me, it was a stratagem.

Both You tried to trick me and it was a stratagem are sentences.

So you need to use a full stop instead of a comma for each sentence.

Is my explanation about 'comma splice' clear?

Bes

To make it even clearer: a comma-splice sentence is a mistake in ordinary writing, though it is sometimes used to good effect in literature and rhetoric.