diff --git a/lib/warbler/config.rb b/lib/warbler/config.rb
index 01dad970..f5f26d98 100644
--- a/lib/warbler/config.rb
+++ b/lib/warbler/config.rb
@@ -31,7 +31,7 @@ class Config
     # Traits: an array of trait classes corresponding to
     # characteristics of the project that are either auto-detected or
     # configured.
-    attr_accessor :traits
+    attr_reader :traits
 
     # Directory where the war file will be written. Can be used to direct
     # Warbler to place your war file directly in your application server's
@@ -183,8 +183,8 @@ class Config
     attr_reader :warbler_templates
     attr_reader :warbler_scripts
 
-    def initialize(warbler_home = WARBLER_HOME)
-      super()
+    def initialize(warbler_home = WARBLER_HOME, forced_traits: [])
+      super(forced_traits)
 
       @warbler_home      = warbler_home
       @warbler_templates = "#{WARBLER_HOME}/lib/warbler/templates"
diff --git a/lib/warbler/traits.rb b/lib/warbler/traits.rb
index 17d3499a..78c5abc9 100644
--- a/lib/warbler/traits.rb
+++ b/lib/warbler/traits.rb
@@ -15,14 +15,28 @@ module Warbler
   # the kind of project and how it should be packed into the jar or
   # war file.
   module Traits
-    attr_accessor :traits
-
-    def initialize
+    def initialize(forced_traits = [])
+      @forced_traits = forced_traits
       @traits = auto_detect_traits
     end
 
     def auto_detect_traits
-      TraitsDependencyArray.new(Traits.constants.map {|t| Traits.const_get(t)}).tsort.select {|tc| tc.detect? }
+      all_trait_classes = Traits.constants.map { |t| Traits.const_get(t) }
+      sorted = TraitsDependencyArray.new(all_trait_classes).tsort
+
+      conflicts = @forced_traits.flat_map { |ft| ft.conflicts }.uniq
+
+      sorted.select do |tc|
+        if @forced_traits.include?(tc)
+          true
+        elsif conflicts.include?(tc)
+          false
+        elsif tc.requirements.any? && tc.requirements.all? { |req| conflicts.include?(req) }
+          false
+        else
+          tc.detect?
+        end
+      end
     end
 
     def before_configure
@@ -53,6 +67,10 @@ module ClassMethods
       def requirements
         []
       end
+
+      def conflicts
+        []
+      end
     end
 
     def self.included(base)
diff --git a/lib/warbler/traits/jar.rb b/lib/warbler/traits/jar.rb
index b9690403..e6ac5ef6 100644
--- a/lib/warbler/traits/jar.rb
+++ b/lib/warbler/traits/jar.rb
@@ -20,6 +20,10 @@ def self.detect?
         !War.detect?
       end
 
+      def self.conflicts
+        [War]
+      end
+
       def before_configure
         config.gem_path      = '/'
         config.pathmaps      = default_pathmaps
diff --git a/lib/warbler/traits/war.rb b/lib/warbler/traits/war.rb
index a94af5cd..bd74a01b 100644
--- a/lib/warbler/traits/war.rb
+++ b/lib/warbler/traits/war.rb
@@ -18,7 +18,11 @@ class War
       DEFAULT_GEM_PATH = '/WEB-INF/gems'
 
       def self.detect?
-        Traits::Rails.detect? || Traits::Rack.detect?
+        Rails.detect? || Rack.detect?
+      end
+
+      def self.conflicts
+        [Jar]
       end
 
       def before_configure
diff --git a/spec/warbler/traits_spec.rb b/spec/warbler/traits_spec.rb
index 0e3ff8e3..0401b85f 100644
--- a/spec/warbler/traits_spec.rb
+++ b/spec/warbler/traits_spec.rb
@@ -18,4 +18,62 @@
       end
     end
   end
+
+  describe "conflicts" do
+    it "Jar conflicts with War" do
+      expect(Warbler::Traits::Jar.conflicts).to include(Warbler::Traits::War)
+    end
+
+    it "War conflicts with Jar" do
+      expect(Warbler::Traits::War.conflicts).to include(Warbler::Traits::Jar)
+    end
+
+    it "traits with no declared conflicts return an empty array" do
+      expect(Warbler::Traits::Bundler.conflicts).to eq([])
+    end
+  end
+
+  describe "forced_traits" do
+    context "in a Rack project with Bundler" do
+      run_in_directory 'spec/sample_bundler'
+
+      it "auto-detects War trait by default" do
+        config = Warbler::Config.new
+        expect(config.traits).to include(Warbler::Traits::War)
+        expect(config.traits).to include(Warbler::Traits::Rack)
+        expect(config.traits).to_not include(Warbler::Traits::Jar)
+      end
+
+      it "forces Jar and excludes War when Jar is forced" do
+        config = Warbler::Config.new(forced_traits: [Warbler::Traits::Jar])
+        expect(config.traits).to include(Warbler::Traits::Jar)
+        expect(config.traits).to_not include(Warbler::Traits::War)
+      end
+
+      it "excludes traits that require an excluded trait" do
+        config = Warbler::Config.new(forced_traits: [Warbler::Traits::Jar])
+        expect(config.traits).to_not include(Warbler::Traits::Rack)
+      end
+
+      it "preserves non-conflicting auto-detected traits" do
+        config = Warbler::Config.new(forced_traits: [Warbler::Traits::Jar])
+        expect(config.traits).to include(Warbler::Traits::Bundler)
+      end
+
+      it "runs before_configure with forced traits" do
+        config = Warbler::Config.new(forced_traits: [Warbler::Traits::Jar])
+        expect(config.jar_extension).to eq('jar')
+      end
+    end
+
+    context "with no forced traits" do
+      run_in_directory 'spec/sample_jar'
+
+      it "behaves identically to auto-detection" do
+        default_config = Warbler::Config.new
+        forced_config = Warbler::Config.new(forced_traits: [])
+        expect(forced_config.traits).to eq(default_config.traits)
+      end
+    end
+  end
 end