It doesn't help to look at the singular form alone. You must take in account the declension of the plural form too.
The Wikipedia article about German declension lists »Staat« as an example for gemischte Deklination. This word is listed as example for the class W3: 1. Mischklasse, Genitiv -(e)s. It belongs to this class because it is declined strong in the singular form by adding »-es« to the singular genitive, but is declined weak in the plural form by adding »-en« to all four cases:
Fall |
Singular |
Plural |
Nominativ |
der Staat |
die Staaten |
Genitiv |
des Staates |
der Staaten |
Dativ |
dem Staat |
den Staaten |
Akkusativ |
den Staat |
die Staaten |
But der Staat is one of those words that had more than just one plural forms in former days. (Most prominent example of a word that still has multiple plural forms: das Wort - die Worte, die Wörter.) In fact, »der Staat« even had four different plural forms, but three of them became distinct more than 200 years ago. Wiktionary lists all of them, and I copied the full declension table here:
Fall |
Singular |
Plural 1 |
Plural 2 |
Plural 3 |
Plural 4 |
Nom |
der Staat |
die Staaten |
die Stäte |
die Staat |
die Staate |
Gen |
des Staates |
der Staaten |
der Stäte |
der Staat |
der Staate |
Dat |
dem Staat |
den Staaten |
den Stäten |
den Staaten |
den Staaten |
Akk |
den Staat |
die Staaten |
die Stäte |
die Staat |
die Staate |
- Plural 1 is mixed declension (class W3), as exlained before
Other words in this class: der Schmerz, der Autor
- Plural 2 is strong declension (class S1: plural with umlauts and -e)
Other words in this class: der Baum, der Bach
- Plural 3 is strong declension (class S6: unchanged plural except dative -en)
Other words in this class: der Teufel, der Meister
- Plural 4 is strong declension (class S4: plural with -e and dative -en)
Other words in this class: der Berg, der Fisch
Since about 1800 only the plural 1 is used, the three other forms are only of historic interest and have become completely distinct.
Note, that there are also alternative forms for the singular of »Staat«, but they don't influense the class of declension:
Genitive case:
Both form are in use and they mean the same. The frequency of »des Staats« is growing while »des Staates« becomes less used over the years, but »des Staates« still the more frequently used version.
Dative case:
The form »dem Staate« is outdating and only very rarely used today. It was more often used in the past. But both forms mean the same.